Anim: add new keytype 'generated' styling to the Sequencer theme

Add the color for the new keytype 'generated' to the VSE theme settings
as well.

This includes the versioning code that loads the default from the scene.
Without this, the preference would be initialised to black.

Note that the 'Blender Light' theme does not have any colors for any of
the key types, so I also didn't add the 'generated' type there.

Pull Request: https://projects.blender.org/blender/blender/pulls/123005

release/datafiles/userdef/userdef_default_theme.c

@@ -686,11 +686,13 @@ const bTheme U_theme_default = {
     .keytype_breakdown = RGBA(0xb3dbe8ff),
     .keytype_jitter = RGBA(0x94e575ff),
     .keytype_movehold = RGBA(0x808080ff),
+    .keytype_generated = RGBA(0x585858ff),
     .keytype_keyframe_select = RGBA(0xffbe33ff),
     .keytype_extreme_select = RGBA(0xf28080ff),
     .keytype_breakdown_select = RGBA(0x54bfedff),
     .keytype_jitter_select = RGBA(0x61c042ff),
     .keytype_movehold_select = RGBA(0xffaf23ff),
+    .keytype_generated_select = RGBA(0xa28962ff),
     .keyborder = RGBA(0x000000ff),
     .keyborder_select = RGBA(0x000000ff),
   },