Make 3.0 parallel-installable to 2.x

In particular, rename

  - libraries to lib*-3.0.so
  - pc files to *-3.0.pc
  - include paths to /usr/include/gtk-3.0/*
  - module paths to /usr/lib/gtk-3.0/*
  - rc files names to gtk-3.0/gtkrc
  - commandline utilities to *-3.0
  - adjust documentation

Also change the install location for unix-print headers to
/usr/include/gtk-3.0/unix-print/gtk.

demos/gtk-demo/Makefile.am

@@ -1,7 +1,7 @@
 ## Makefile.am for gtk+/demos
 include $(top_srcdir)/Makefile.decl
 
-democodedir=$(datadir)/gtk-2.0/demo
+democodedir=$(datadir)/gtk-$(GTK_API_VERSION)/demo
 
 ## These should be in the order you want them to appear in the
 ## demo app, which means alphabetized by demo title, not filename
@@ -99,7 +99,7 @@ IMAGEFILES=	alphatest.png		\
 		gnome-gmush.png		\
 		gnome-gsame.png		\
 		gnu-keys.png		\
-		gtk-logo-rgb.gif		
+		gtk-logo-rgb.gif
 
 democode_DATA = $(demos) $(IMAGEFILES) demo.ui