Blender Git Loki

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 worked 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 (used to
act on entire triple only).
* 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.

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

Also, this improves/adds comments for related code.

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

Reviewed by: Sybren St�vel, William Reynish