VSE: Update Strip Modifier UI

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

release/datafiles/userdef/userdef_default_theme.c

@@ -275,6 +275,7 @@ const bTheme U_theme_default = {
     .panel_outline = RGBA(0xffffff11),
     .panel_title = RGBA(0xe6e6e6ff),
     .panel_text = RGBA(0xe6e6e6ff),
+    .panel_active = RGBA(0x4772b3ff),
   },
   .common = {
     .anim = {
@@ -327,7 +328,6 @@ const bTheme U_theme_default = {
     .header_text_hi = RGBA(0xffffffff),
     .tab_back = RGBA(0x1d1d1d00),
     .button = RGBA(0x303030ff),
-    .active = RGBA(0x4772b3ff),
     .vertex_size = 3,
     .outline_width = 1,
     .facedot_size = 4,