VSE: Keyframing in Preview

This PR makes it easier to add keyframes for strips in preview.
This works same way as in 3D viewport, using keying sets. Pressing I
key adds keyframe to default keying set, pressing K dhows menu with
available keying sets. For VSE, location, rotation and scale properties
are available for now. Other existing keying sets are not valid for
VSE.

Deleting keyframes and potentially adding more keying sets will be
handled in separate PR.

Pull Request: https://projects.blender.org/blender/blender/pulls/140107

scripts/startup/bl_ui/space_sequencer.py

@@ -975,6 +975,17 @@ class SEQUENCER_MT_strip_show_hide(Menu):
         layout.operator("sequencer.mute", text="Hide Unselected").unselected = True
 
 
+class SEQUENCER_MT_strip_animation(Menu):
+    bl_label = "Animation"
+
+    def draw(self, _context):
+        layout = self.layout
+
+        layout.operator("anim.keyframe_insert", text="Insert Keyframe")
+        layout.operator("anim.keyframe_insert_menu", text="Insert Keyframe with Keying Set").always_prompt = True
+        layout.operator("anim.keying_set_active_set", text="Change Keying Set...")
+
+
 class SEQUENCER_MT_strip_input(Menu):
     bl_label = "Inputs"
 
@@ -1131,6 +1142,8 @@ class SEQUENCER_MT_strip(Menu):
             layout.separator()
             layout.operator("sequencer.preview_duplicate_move", text="Duplicate")
             layout.separator()
+            layout.menu("SEQUENCER_MT_strip_animation")
+            layout.separator()
             layout.menu("SEQUENCER_MT_strip_show_hide")
             layout.separator()
             if strip and strip.type == 'TEXT':
@@ -3177,6 +3190,7 @@ classes = (
     SEQUENCER_MT_strip_retiming,
     SEQUENCER_MT_strip_text,
     SEQUENCER_MT_strip_show_hide,
+    SEQUENCER_MT_strip_animation,
     SEQUENCER_MT_strip_input,
     SEQUENCER_MT_strip_lock_mute,
     SEQUENCER_MT_image,