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.

modules/other/gail/Makefile.am

@@ -6,7 +6,7 @@ if PLATFORM_WIN32
 no_undefined = -no-undefined
 endif
 
-moduledir = $(libdir)/gtk-2.0/modules
+moduledir = $(libdir)/gtk-3.0/modules
 module_LTLIBRARIES = libgail.la
 
 gail_c_sources =			\