* Added the package guiSupport for custom preferences and other custom
GUI elements, and populated it (moved some classes)
* Added an ExtendedDialogPreference with 3 buttons (negative, neutral
and positive)
* Changed SliderPreference: it now extends ExtendedDialogPreference
instead of DialogPreference
* Implemented a 'default' button for the SliderPreference: it resets the
SeekBar to its default value, and only saves when the positive button is
clicked.