Modifier: add support for vertex groups

Allow blending the imported cache with the modifiers stack above the
MeshCache modifier.

This is particularly useful for instance when dealing with cloth
simulations performed in another software, where some parts of the cloth
are completely pinned (non-simulated, following the armature). Indeed,
this would allow modifying the animation in some areas without having to
rebake the other parts or the cloth, resulting in a much more flexible
workflow.

Reviewed By: #modeling, campbellbarton, mont29

Ref D9898

Fix T89033: segfault reordering animation channels

Fix segmentation fault that can occur when reordering animation
channels.

Under some specific conditions, the list "act->curves" is empty in the
"join_groups_action_temp" function. In particular, this happens when a
scene contains an action that has not been pushed down, and with no
keyframe in it.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D11569

Fix T89033: segfault reordering animation channels

Fix segmentation fault that can occur when reordering animation
channels.

Under some specific conditions, the list "act->curves" is empty in the
"join_groups_action_temp" function. In particular, this happens when a
scene contains an action that has not been pushed down, and with no
keyframe in it.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D11569

Fix T89033: segfault reordering animation channels

Fix segmentation fault that can occur when reordering animation
channels.

Under some specific conditions, the list "act->curves" is empty in the
"join_groups_action_temp" function. In particular, this happens when a
scene contains an action that has not been pushed down, and with no
keyframe in it.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D11569

Fix T89033: segfault reordering animation channels

Fix segmentation fault that can occur when reordering animation
channels.

Under some specific conditions, the list "act->curves" is empty in the
"join_groups_action_temp" function. In particular, this happens when a
scene contains an action that has not been pushed down, and with no
keyframe in it.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D11569

Animation: Allow selection of FCurve + its keys

Selection of an FCurve with box/circle select now selects the entire
curve and all its keys:

- Box selecting a curve selects all the keyframes of the curve.
- Ctrl + box selecting of the curve deselects all the keyframes of the
curve.
- Shift + box selecting of the curve extends the keyframe selection,
adding all the keyframes of the curves that were just selected to the
selection.
- In all cases, if the selection area contains a key, nothing is
performed on the curves themselves (the action only impacts the
selected keys).

Reviewed By: sybren, #animation_rigging

Differential Revision: https://developer.blender.org/D11181

Cleanup: Graph Editor, refactor selection operators

Extract initialisation code of box selection into separate functions.

No functional changes.

Reviewed By: zeddb, sybren

Differential Revision: https://developer.blender.org/D9196