Blender Git Loki

Fix T90637: Outliner: VSE context menu options are not working

Some of the enum options in the context menu operations are not
supported for all element types.

`TSE_SEQUENCE`, for example, only supports the `Select` option.

So, populate the enum list dynamically depending on the type.

Also add some calls that were missing for the `TSE_SEQUENCE` type.
(`WM_event_add_notifier` and `ED_undo_push`).