We previously disabled cache, but that may cause excessive
memory usage with big books. We now use a temporary
cache directory, that we clean when no more needed.
So an image file can have a cover in coverbrowser's display.
It also allows for an alternative viewer (ImageViewer widget)
when holding on file / View full size cover.
* CoverBrowser plugin: alt views for File Browser and History
* Added Prune cache and Compact cache menu actions
* Support for book descriptions, and settings stored in db
