Demos: Fix Demo Data Path on Windows

The relocatable data path for the gtkmm demos was still set in the
gtkmm-2.x days.  Update the path so that gtkmm-2.x and gtkmm-3.x can be
really parallel-installed on Windows in the same root path.

	* demos/gtk-demo/demo-common.cc: Update the relocatable demo data
	  path for Windows so that we don't have to fight for the same
	  space with gtkmm-2.x.

https://bugzilla.gnome.org/show_bug.cgi?id=736846