Alembic: fix updating of GUI when export progress is made

The `do_update` variable wasn't set after changing the `progress`
variable, causing the GUI only to update on redraw (f.e. when the user
was waving the mouse around).

Cleanup: Alembic: use pointers instead of references when passing progress variables

Using pointers instead of references when passing progress variables
makes the C++ code more in line with the C code (as it doesn't transform
pointer parameters to reference parameters). Also makes it easier to
spot when a common Blender pattern is implemented incorrectly (fix will
be in the next commit).

User Preferences: Added "Enabled add-ons only" preference

This checkbox replaces the "Disabled" and "Enabled" entries in the
filter drop-down. As a result, it now takes a single click to limit the
shown entries to enabled add-ons only. This is also an actual flag in
the preferences, and thus its state is saved between runs on Blender (in
contrast to the filter, which is always reset to "All").

Reviewed by: brecht, billreynish

Fix T56408: Hair children recalc on every frame on Alembic mesh

This fixes the glitching hairs described in T56408, T63534, and possibly
also T63534.

The fix consists of returning the original mesh (i.e. as visible in edit
mode) when constructing the ORCO mesh. This allows a static set of
coordinates to be used when computing the child hair positions.

The original mesh is only returned when it has the same topology (at
least same number of vertices, loops, and polys. It's up the author of
the Alembic file to ensure stable geometry when it's desired to be
compatible with Blender's hair system.

Reviewers: mont29, brecht

Differential Revision: https://developer.blender.org/D5492

Fix T67999: calling Mesh.materials.clear() crashes Blender

The `BKE_material_pop_id()` and `BKE_material_clear_id()` functions had
a parameter `update_data` that, when `false`, would cause the mesh polys
to keep their material index, even when the indexed material slots were
removed. This behaviour was never used in the C code and not supported
by the drawing code, making polygons disappear and causing crashes. The
Python binding in RNA, however, defaulted to `update_data=False`.

This commit removes the `update_data` parameter altogether, and makes
the functions always fix up the material indices.

Reviewed by: mont29, brecht

Text editor: syntax highlighting + line numbers on by default

The most common use of the text editor seems to be for scripting. Having
line numbers and syntax highlighting enabled by default seems sensible.

Syntax highlighting is now enabled by default, but is automatically
disabled when the datablock has a non-highlighted extension.
Highlighting is enabled for filenames like:
- Text
- Text.001
- somefile.py
and is automatically disabled when the datablock has an extension for
which Blender has no syntax highlighter registered.

Reviewers: billreynish, campbellbarton

Subscribers: brecht, billreynish

Differential Revision: https://developer.blender.org/D5472

Added missing forward declaration

Fix T67516 VSE: Animation evaluated incorrectly when scene strip present

The calls to `BKE_animsys_evaluate_all_animation()` and
`BKE_mask_evaluate_all_masks()` used the wrong timecode to evaluate the
animation system. This happened:

- Sequencer in scene A was rendered at frame X.
- Scene strip for scene B which should be evaluated at frame Y.
- BKE_animsys_evaluate_all_animation() was called with frame Y, which
also re-evaluated the animation data in scene A.
- Other sequencer strips with animated values were then evaluated for
frame Y and not frame X.

Since the depsgraph for rendering the scene strip is already OK and does
its job, it's no longer necessary to re-evaluate all the animation in
this way.

Removed `BKE_mask_evaluate_all_masks()` because it's no longer used.

Reviewers: sergey, brecht, iss

Differential Revision: https://developer.blender.org/D5394

Fix T68322: Shear in Dopesheet causes crash

The Shear transform operator is now disallowed in the timeline and
dopesheet editors.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D5440

Clarify "Save on Exit" tooltip

The old text, "Save modified preferences on exit" suggests that only the
modified preferences are saved. This is not the case: all preferences
are saved at once. This distinction is especially important after having
loaded factory default settings.

As discussed with @campbellbarton and @JulienKaspar.