VSE: Add option to translate pivot point

This feature allows you to change postion of origin/pivot for images
without changing their position.

It is implemented as property of transform operator. It is activated
by pressing `Ctrl + .` shortcut.
Move Origin item was also added to transform menu.

Origin can be snapped to 3x3 grid on strip image. This represents
most usual anchor points.

Ref: #134251
Pull Request: https://projects.blender.org/blender/blender/pulls/134206

scripts/startup/bl_ui/space_sequencer.py

@@ -1180,6 +1180,8 @@ class SEQUENCER_MT_image_transform(Menu):
         layout.operator("transform.translate")
         layout.operator("transform.rotate")
         layout.operator("transform.resize", text="Scale")
+        layout.separator()
+        layout.operator("transform.translate", text="Move Origin").translate_origin = True
 
 
 class SEQUENCER_MT_image_clear(Menu):