Benoit Touchette authored Aug 23, 2016 and Michael Natterer committed Aug 23, 2016

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