Markers: Make marker lines in sequencer and graph editor optional

- Makes it possible to show a vertical line for every marker in the graph editor.
- Makes the marker line visiblity optional in the sequencer and graph editor.

Request from @hjalti.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D4348

release/scripts/startup/bl_ui/space_sequencer.py

@@ -212,6 +212,7 @@ class SEQUENCER_MT_view(Menu):
             layout.prop(st, "show_seconds")
             layout.prop(st, "show_frame_indicator")
             layout.prop(st, "show_strip_offset")
+            layout.prop(st, "show_marker_lines")
 
             layout.prop_menu_enum(st, "waveform_display_type")