theme: support theme setting override

...by reading <config-dir>/themerc-override where <config-dir> is normally
$HOME/.config/labwc can be other locations as described in labwc-config(5)
and can also be specified by the command line option -C.

The reason for supporting theme override is to give users more fine-
grained control of settings without making local copies and modifying
themes.

docs/meson.build

@@ -30,6 +30,7 @@ install_data(
     'environment',
     'menu.xml',
     'README',
+    'themerc',
     'rc.xml',
     'rc.xml.all'
   ],