Playlist context menu: Cover vs Album

[Louis BAYLE]

Hi,

as Mark Kretschmann suggested me in the Merge request
#2254: add "Move to Trash", I'm changing the playlist context-menu
to add a "Cover" sub-menu containing the 4 cover actions.

The cover actions come from the Album CustomActionsCapability,
together with another action "show under various artists" which is
also in the context-menu.
I suggest to make an 'Album' submenu (instead of 'Cover') that
includes the cover actions and the "show under various artists"
action.

reasons are:
- if those are all album actions, why not putting them together
- reduce the context-menu size
- code is much simplified (no need to change CustomActionsCapability)

I made a branch on gitorious with the sub-menu. I'm new to gitorious
so I don't know if the following path is enough for you to pull, but
here it is :
~lbayle/amarok/lbayle-amarok:playlist_submenu_cover

best regards
Louis