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/gtkfilechooser.c

@@ -255,7 +255,7 @@
  * You can change these defaults to something else.  For
  * example, to add a <keycap>Shift</keycap> modifier to a few
  * of the default bindings, you can include the following
- * fragment in your <filename>.gtkrc-2.0</filename> file:
+ * fragment in your <filename>.gtkrc-3.0</filename> file:
  * </para>
  * <programlisting>
  * binding "my-own-gtkfilechooser-bindings" {
@@ -330,7 +330,7 @@
  *    frequently.  You could then create an <keycombo>
  *    <keycap>Alt</keycap> <keycap>M</keycap> </keycombo>
  *    shortcut by including the following in your
- *    <filename>.gtkrc-2.0</filename>:
+ *    <filename>.gtkrc-3.0</filename>:
  *    <programlisting>
  *    binding "misc-shortcut" {
  *       bind "&lt;Alt&gt;M" {