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
Remove unused properties from TabInterface and TabWidget

Not sure what those were supposed to help with.
specify widget,helptext,iconpath of tabs on class itself

helptext is obviously broken, lazy translation missing

Conflicts:
	horizons/gui/tabs/boatbuildertabs.py
	horizons/gui/tabs/buildrelatedtab.py
	horizons/gui/tabs/buyselltab.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