Merge branch 'master' into temp-graph-select-changes

Cleanup: Slight adjustments to comment

Merge branch 'master' into temp-graph-select-changes

Cleanup: Detailed comments on selection helpers & simplify logic

Fix incorrect return value, cleanup return values

Fix incorrect use of use_handle option

Minor cleanup: Clarify logic

Cleanup: Clarify iterator visibility flag

Fix incorrect selection flag usage

Cleanup: Comments, use enum types or aliases in parameter lists

Merge branch 'master' into temp-graph-select-changes

Merge remote-tracking branch 'origin/master' into temp-graph-select-changes

Merge branch 'temp-graph-select-changes' of git.blender.org:blender into temp-graph-select-changes

UI: Changes to graph editor selection and transform

When introducing "drag-all-selected" support all over Blender, we figured this wouldn't work well with the Graph Editor selection/transform behavior.
Hence, William and I have been working on the following changes, although we used this chance to improve the behavior in general too.
For more info see T70634.

* Handles now always move with the key, regardless if they are selected or not.
* Selecting the key doesn't select the handles anymore, their selection is separate.
* Multiple keys and handles can now be dragged.
* Dragging a handle moves all selected handles on the same side.
* Tweak-dragging any handle can never affect any keyframe location, only handles
* G/R/S should behave as before.
* Changing the handle type with a key selected always applies the change to both handles.
* Box selection with Ctrl+Drag now allows deselecting handles.
* Box selection Include Handles option now only acts on visible handles, wasn't the case with Only Selected Keyframes Handles enabled.
* Box selection Include Handles is now enabled by default in all bundled keymaps.

If needed I can give more detailed rationales for each change, but keeping this short.

The changes have been tested for some days by the animators here in the Blender Animation Studio. Some changes are based on their feedback.

Differential Revision: https://developer.blender.org/D6235

Minor cleanup (style, comments)

Always apply handle type change to both handles when key is selected.

Previously we only applied the change to both handles if only the key
but no handle is selected.
Based on feedback by Hjalti, this new behavior is more practicable.
Cases where you'd want only one handle to be changed are really rare (he
can't think of one himself) so rather make the common case work well.

This wasn't much of an issue before, as selecting a key always selected
the handles with it, and box select had `Include Handles` disabled by
default.

Change keymaps to use box select Include Handles by default

Also removes Ctrl+B and Ctrl+Alt+B (Ctrl+Q and Ctrl+Alt+Q in the
industry compatible keymap) which are now redundant in the keymap: they
used to toggle the Include Handlers option only.
We could instead use them to disable Include Handles, but lets first see
if there's much desire for it.

Make box selection Include Handles option only act on visible handles

The option would previously act on invisible handles. Because of this,
it wouldn't be wise to enable it by default, even though it can be
tremendously useful (esp. with `Only Selected Keyframes Handles`
disabled).
This changes things based on animator feedback, so that we can enable
the option by default.

I can see that for some people even the old behavior of acting on
invisible handles could be useful, so we might consider bringing that
option back. `Include Handles` could then become an enum to chose the
behavior.

Merge branch 'master' into temp-graph-select-changes

VSE: open file browser sidebar by default when adding external strips

Open the file browser sidebar by default when adding Movie/Sound/Image Sequence Strips, to show the operator options.