Remove old code related to GOOCANVAS and CLUTTER.

* configure.ac: Remove options to enable CLUTTER and GOOCANVAS support.
* src/Makefile.in: Remove CLUTTER_FLAGS and CLUTTER_LIBS
* src/xwidget.c: Remove code protected by preprocessor conditionals.
* src/xwidget.c: Prefix all Lisp keywords on the C side by QC (standard
convention).

This is done especially for Qtitle which is used here as a keyword but
which is also declared in frame.c and used as a regular symbol.