Remember column resizes

1. Only remembers resizes for columns in RBEntryView, if requested.

2. All playlists have individual column widths.

3. All other sources share column widths with their peers / subsources ( eg. all Audio CD's have same column widths )

4. Sets minimum width for columns in RBEntryView based on column title.

5. Handles 2 modes of loading resized column widths "resized" ( for non-playlists source views ) and "resized-delayed" for playlist source views ( static playlists / automatic playlists / play queue )

6. "resized" mode is when the resized column widths are available when the view is created. This mode is used by non-playlist based sources ( Podcast / Library / Grilo / Magnatune etc )

7. "resized-delayed" mode is when the resized column widths are not available when the view is created. This is due to the fact that per playlist widths are saved in 'playlists.xml` file which needs to be loaded past source creation.

8. Test to check load and save on column resize.

Fixes #409

Column resize remember - UI test: