Prelim commit to loading screen.

This should be rebased into a single commit including any
other updates needed.
Move WorldLoadingScreen to Ember::OgreView::Gui namespace.
Remove inline access to impl file
Remove unused includes
Remove GUIManager logic for when to display loading screen.

Instead, push this functionality onto the class itself, so that
it knows when to display itself.  This is done by subscribing
to the same signals that the gui manager does.
Change window to be EmberLook/StaticText.

Change from Framewindow.
Drop forward declarations into header.