Bug 759601 - Add/Remove layer masks by clicking the layer preview

Quite heavily modified by Mitch to address the more general issues
mentioned in comment 35 of the bug:

- Remember the added mask's type and the invert boolean in
  GimpDialogConfig
- Add new prefs page "Dialog Defaults" which will contain
  only stuff from GimpDialogConfig and can be reset separately
- Remove static mask variables from layers-commands.c and
  use the new config values for both interactive mask adding
  with the dialog, and for the shortcut based method
- Add a button to the layers dialog which supports add, add
  with last values, delete, apply
- Add modifier-click shortcuts on the layer preview with the
  same modifiers as on the button