Kovid Goyal a43941dc0d 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

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dark_cloth.png

Allow using textures for cover grid background

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dark_wood.png

Allow using textures for cover grid background

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grey_wash_wall.png

Allow using textures for cover grid background

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light_wood.png

Allow using textures for cover grid background

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subtle_wood.png

Allow using textures for cover grid background

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