Add ability to fit now playing cover to width

Some people seem to want to have a way of enlarging the now playing
widget cover when in large mode to get rid of the black bars on the sides.
This patch adds an option in the context menu which allows the cover to
fill the width of the panel when either large mode is enabled.
Use UI background colour when in fit width mode

When in fit width mode with the details below the artwork,
drawing the black background is out of place. This uses the ui colour
making it less obtrusive.
Set text size on now playing widget

In removing the background colour I accidentally removed the text size style.
This edit re-adds it so the size is uniform against all modes.
Initialize fit_width_
Merge pull request #4350 from TheUbuntuGuy/master

Add ability to fit now playing cover to width