Draft: Multiple subtitle tracks

This merge is target to fix/update #1022 (closed). This will allow for multiple subtitle tracks for multiple languages in the same video. This is currently a unchanged branch so far but will be used to make this change

• Use a proper id for the subtitle track instead of -2, add a Subtitle track type here
• Add methods to create / delete a subtitle tracks, each track using its own subtitlemodel
• Handle the avfilter.subtitle effects to put them in the correct order
• Refactor code that is currently using subtitlemodel in timelinemodel.cpp, timelinecontroller.cpp and core.cpp to handle multiple tracks (store a list of subtitletracks in timelinemodel ?)
• UI: Move subtitle track stuff outside of timeline.qml into its own files to make it usable for several tracks
• UI: Update styling to ensure it applies to the active track
• UI: Allow renaming a subtitle track
• Handle loading / saving