Add single document support back into the builder.  This is off by default.
Add single document support back into the builder.  This is off by
default.

gui/view_manager.py:
	Added a set_single_document_mode() function that puts the tab widget in
   a state that closes vistrails before opening a new one and hides the
   tab bar.




minesense/VisTrails,Nikea/VisTrails,celiafish/VisTrails,celiafish/VisTrails,minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,Nikea/VisTrails,hjanime/VisTrails,celiafish/VisTrails,celiafish/VisTrails,minesense/VisTrails,Nikea/VisTrails,minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,Nikea/VisTrails,hjanime/VisTrails