Sequencer: srt export support.

This commit adds a new operator that will compile the list of text
strips into an srt file. No positioning is supported yet but will
be added later.

The operator can be found in the effect panel in the strip properties.

release/scripts/startup/bl_ui/space_sequencer.py

@@ -639,6 +639,7 @@ class SEQUENCER_PT_effect(SequencerButtonsPanel, Panel):
             if not strip.use_autocenter:
                 row.prop(strip, "xpos")
             row.prop(strip, "ypos")
+            layout.operator("sequencer.export_subtitles")
 
         col = layout.column(align=True)
         if strip.type == 'SPEED':