Edit Book: Allow typing arbitrary unicode characters with the keyboard in the editor

To use this feature, just type the unicode code number and press Alt+X.
For example, to get the character ÿ type ff and press Alt+X. Or to get
the character 🐱 type 1f431 and press Alt+X

Fixes #1280868 [Windows Alt+ key input does not accept hex digits A-F](https://bugs.launchpad.net/calibre/+bug/1280868)