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

source/blender/editors/space_graph/space_graph.c

@@ -322,7 +322,11 @@ static void graph_main_region_draw(const bContext *C, ARegion *ar)
 
 	/* markers */
 	UI_view2d_view_orthoSpecial(ar, v2d, 1);
-	ED_markers_draw(C, DRAW_MARKERS_MARGIN);
+	int marker_draw_flag = DRAW_MARKERS_MARGIN;
+	if (sipo->flag & SIPO_MARKER_LINES) {
+		marker_draw_flag |= DRAW_MARKERS_LINES;
+	}
+	ED_markers_draw(C, marker_draw_flag);
 
 	/* preview range */
 	UI_view2d_view_ortho(v2d);