Unify interface for normal and lazy loading of tab widgets

Conflicts:
	horizons/gui/tabs/boatbuildertabs.py
	horizons/gui/tabs/buildrelatedtab.py
	horizons/gui/tabs/diplomacytab.py
	horizons/gui/tabs/inventorytab.py
	horizons/gui/tabs/mainsquaretabs.py
	horizons/gui/tabs/overviewtab.py
	horizons/gui/tabs/selectmultitab.py
	horizons/gui/tabs/shiptabs.py
	horizons/gui/tabs/tabinterface.py
	horizons/gui/tabs/tradetab.py