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.

gtk+-3.0-uninstalled.pc.in

@@ -0,0 +1,10 @@
+target=@gdktarget@
+
+gtk_binary_version=@GTK_BINARY_VERSION@
+
+Name: GTK+ Uninstalled
+Description: GTK+ Graphical UI Library (${target} target), Not Installed
+Version: @VERSION@
+Requires: gdk-${target}-@GTK_API_VERSION@-uninstalled @GTK_PACKAGES@
+Libs: ${pc_top_builddir}/${pcfiledir}/gtk/libgtk-${target}-@GTK_API_VERSION@.la @GTK_EXTRA_LIBS@
+Cflags: -I${pc_top_builddir}/${pcfiledir}/@srcdir@ -I${pc_top_builddir}/${pcfiledir} @GTK_EXTRA_CFLAGS@