Cache video media lists and update according to model notifications

In master, we currently fetch the entire media library list for videos every time one of the collections-specific methods for VLCLibraryVideoDataSource gets called.

This MR caches the library video list and only updates it when the model sends out a notification that the video list has changed, which should prevent unnecessary resource usage.

Signed-off-by: Claudio Cambra developer@claudiocambra.com