WB-842 Opening a spec or scene file from Finder on MacOS Mojave (10.14) does...

WB-842 Opening a spec or scene file from Finder on MacOS Mojave (10.14) does not work: This works by the MacApplication class receiving a QFileOpenEvent and then passing it along through the Workbench event system to GuiManager.  It seems that in MacOS Mojave, this is happening very quickly, before GuiManager is created.  So, move the creation of GuiManager to just after the creation MacApplication.