Use QStackedWidget instead of manual widget showing/hiding

Thanks to Thomas Lübking for reminding me that the widgets have to be explicitly
shown after reparenting.
GUI: disable the wide mode on small screens

Suggested by Thomas Lübking.
GUI: wait a bit before reacting to screen resolution changes
GUI: enable sizes and positions of various widgets

Fully separate settings are maintained for the wide and compact layout modes.
Nothing is saved/restored when in the one-at-time mode.
GUI: also save & restore the sizes and state of the header fields in the message list
GUI: keep the actions for disabling/enabling headers in action with their actual state when restoring
GUI: resize the header columns *initially*, but repsect the user preferences later on