Added left-click to show fullsize cover on NowPlayingWidget.

This intuitively mirrors the behaviour of clicking an image in the Artist Info page
Changes as requested

Removed unnecessary () and changed to hypnotoad_.get()
Merge pull request #4416 from alecksphillips/click-show-cover

Added left-click to show fullsize cover on NowPlayingWidget.
Add a tooltip to this new option. IMO the text description might be not obvious enough to everyone
Merge pull request #2 from clementine-player/master

update