enhanced SceneToolBar tool handling (remove and insert at specific point)
added ModeButton specialisation of PushButton for use in SceneToolMode
allow externally supplied ModeButtons in SceneToolMode
renamed getElementMask to getVisibilityMask
added edit mode button to scene toolbar
added edit modes for exteriors
Merge branch 'mode'

Conflicts:
	apps/opencs/view/render/pagedworldspacewidget.cpp
	apps/opencs/view/render/worldspacewidget.cpp
	apps/opencs/view/render/worldspacewidget.hpp
use placeholder icons in OpenCS when no proper icon is available