VLCServerListViewController: Fix the local files playback

This merge request brings a fix to the several behaviours of the local files playback.

The user is now able to select one or multiple files and hit the "Open" button in order to start the playback.

The user can also hit the "Open" button without having any media selected in order to start the playback of the entire folder.

This merge request also adds an alert that is displayed to notify the user the media list is currently empty making it impossible to start the playback.

Screen recordings

Local_files_-_New_behaviours Local_files_-_Empty_Media_List_Alert