This replaces the "Add modifier" type enum UI with a menu,
similar to the object modifiers "Add modifier" menu.
The "Copy to Selected Strips" operator was a bit out of place
and already exists in the "Strip" menu.
Togther with the "Add" menu, these are now moved into a
"Modifiers" submenu, similar to the "Objects" > "Modifiers" menu.
Also updated for the strip context menu (right-click).
Pull Request: https://projects.blender.org/blender/blender/pulls/145981
8dcd01907d