UI: allow to hide markers region per editor

Instead of having the option to show marker lines,
make the marker region optional.

- Added a Show Markers entry in the View menu of the animation editors.
- If the markers region is not active then the Marker menu gets hidden.
- Removed marker menu from the driver editor
  and don't allow to use marker operators.

release/scripts/startup/bl_ui/space_sequencer.py

@@ -134,7 +134,8 @@ class SEQUENCER_MT_editor_menus(Menu):
 
         if st.view_type in {'SEQUENCER', 'SEQUENCER_PREVIEW'}:
             layout.menu("SEQUENCER_MT_select")
-            layout.menu("SEQUENCER_MT_marker")
+            if st.show_markers:
+                layout.menu("SEQUENCER_MT_marker")
             layout.menu("SEQUENCER_MT_add")
             layout.menu("SEQUENCER_MT_strip")
 
@@ -268,7 +269,8 @@ class SEQUENCER_MT_view(Menu):
 
             layout.prop(st, "show_seconds")
             layout.prop(st, "show_strip_offset")
-            layout.prop(st, "show_marker_lines")
+            layout.separator()
+            layout.prop(st, "show_markers")
 
         if is_preview:
             layout.separator()