This new interface is based on a suggestion by Yves-Alexis Pérez. The
former main dialog is split into two dialogs: one for selecting the
region to be captured and the delay, while the second one displays a
preview of the screenshot and lists the available actions.

The main application shows the first dialog, then the second one. If
one of the region cli options is given, the screenshot is taken
accordingly and the second dialog is displayed.

The panel plugin uses the first dialog as a configuration dialog. When
you click the plugin, the screenshot is taken and the second dialog is
shown.