Valama: Improved GUI - general gnomeification ;) (Loosely following design proposal https://live.gnome.org/Design/Apps/IDE)

 * Deactivated menu bar (Once the about dialog is added to the welcome screen, it should be obsolete)
 * Allow returning to welcome screen (Makes open project button obsolete)
 * Initial work on a GUI lock (GDL still causes some drawing trouble)
 * Abstracted UIElements' public widget (Necessary for lock), UIElements now hold a reference to their Gdl.DockItems