Fix error if a marked book is deleted/merged.

Fixes #1239161 [Unhandled exception using "Mark books" feature](https://bugs.launchpad.net/calibre/+bug/1239161)
Allow marked book button to be added to main toolbar when device is connected

Fixes #1239163 [Mark Books cannot be added to "The main menu when a device is connected"](https://bugs.launchpad.net/calibre/+bug/1239163)
Get Books: Fix downloading of some books in formats that do not have metadata yielding nonsense titles

Merge branch 'master' of https://github.com/user-none/calibre
Fix crashes when switching libraries with marked books

Fix a bug that could cause calibre to crash when switching from a large
library to a smaller library with marked books. Fixes #1239210 [Calibre 1.6 64bits crashes when changing library](https://bugs.launchpad.net/calibre/+bug/1239210)
Update Diagonales

Fixes #1239235 [Updated recipe for Diagonales](https://bugs.launchpad.net/calibre/+bug/1239235)
Start work on global undo/redo for tweaking
Update Carta
Update Television Without Pity