This commit also fixes a segmentation fault that would be caused when deleting libraryWidget (Gutenberg) that first appeared in 4a104522.

TODO: Handle what happens when there are no books in the library
TODO: Re-generate database upon exiting USBMS session or books download (libraryWidget)
TODO: Adjust various icon width/height variables depending on the device

Thanks to @shermp for the help provided on QVectors, which I now use here instead of C-style arrays which could only have a fixed size.

A local books database is maintained at `/data/onboard/.inkbox/LocalLibrary.db`. It contains compressed JSON read by the GUI.

URGENT TODO: Fix `*** Error in '/mnt/onboard/.adds/inkbox/inkbox-bin': double free or corruption (out): 0x00648708 ***` error whenever we try to delete localLibraryWidget

Main credit to @Szybet for this commit.

Huge thanks to @Szybet for writing at least 95% of the code in this commit

Apparently log() makes the device hang forever on the "Sync in progress" screen. So no option but to remove those calls ...