Basic Subtitling Support in Kdenlive

This issue shall be used for suggestions and ideas for adding basic Subtitling Support in Kdenlive.

Ideas / TODO for 20.12.0 release:

• Update nightly appimage dependencies
• Trigger monitor refresh when a subtitle is changed [jbm]
• Allow selecting a subtitle item [jbm]
• Integrate with undo/redo [jbm]
• Better color scheme for subtitles
• Add subtitle item context menu (duplicate, delete, ...)
• Fix mouse scroll / zoom while mouse is on the subtitle track [jbm]
• When double clicking to edit a subtitle, show a larger edit line to ensure all text is visible. (Implemented as subtitle window widget.)
• Track head: add "Subtitle Track" label and options like "Lock Track" and "Disable Track" as on the other track types
• Windows: no subtitle text visible on the project monitor while playback.
• Click on "edit subtitle tool" to close the subtitle track: It should no delete all subtitles.