TabBarCoordinator: Retain and restore the last used tab when VLC starts

This commit makes VLC store the last used tab into User Defaults, which is a hack.

I tried using Apple state retainment API, see Preserving Your App's UI Across Launches, but could not restore the state after saving it.
I think the TabBarCoordinator needs some rework to :

• inherit UIViewControllerRestoration;
• override decodeRestorableState and encodeRestorableState correctly.