qt/qml: Improve Tracks panel design and implementation

This MR seeks to improve the "Tracks" panel while retaining the previous functionnalities. We now have a dedicated QMenu for Subtitle and Audio and a '+' button to open a custom video track.

The panel has the following sub-pages:

• The playback speed page
• The subtitle synchronize page
• The audio synchronize page

We are lazy loading subpages and have an automatic resize based on a preferred width.

NOTE: A revamp is scheduled for the playback speed view, that's used both for this panel and the custom playback speed button. Notably with a list of presets à la Youtube.