Tuned the settings dialog a bit, mostly fixing the layout problems we had and adding

Tuned the settings dialog a bit, mostly fixing the layout problems we had and adding
functionality.
It should now be possible to register settings pages and use them in the settings dialog.
A settings page is a QWidget implementing the SettingsInterface as defined in plugin.h.
Currently, these are defined in settingspages.* (which may change... maybe it'd be a good
idea to put all settings related stuff in an extra dir below qtgui/?)