Make Histogram keyboard accessible

Histogram can now be used by keyboard as follows:

- Left/Right key will move the manipulator
- Shift will select which manipulator is used. No Shift => left
  manipulator, Shift => right manipulator
- Normal increase or decrease is 5. Using CTRL this can be modified to 1

https://bugzilla.gnome.org/show_bug.cgi?id=716431