This PR updates the VSE strip modifiers interface.
It now uses the same design as the object modifiers.
Changes:
* Except for the "Mask Input" subpanel, the modifier UIs are unchanged.
* Modifiers can now be rearranged using drag & drop.
* Additionally, there is now an active strip modifier. This is exposed
though python via `strip.modifiers.active`.
This is in part for !139634 which needs the concept of an active modifier.
Notes:
* The `modifier.cc` file included all the implementation of all modifiers.
With the addition of a another new callback in this PR, this file was
getting quite big so I split everything out into individual files for all
modifiers. The modifiers are getting registered at launch.
* The modifier panels are getting added using a UI template
(`template_strip_modifiers`) very similar to the object modifiers.
Pull Request: https://projects.blender.org/blender/blender/pulls/145367
866fcd0a09