Allow using textures for cover grid background

Cover grid: Allow using images as the background for the cover grid. To
choose an image, go to Preferences->Look & Feel->Cover Grid. Fixes #1239194 [[Ideas] -->book grid](https://bugs.launchpad.net/calibre/+bug/1239194) [[Ideas] -->book grid](https://bugs.launchpad.net/calibre/+bug/1239194)

The builtin textures come from subtlepatterns.com