plugin: Add context handler base class

In order to bridge work with GL between GTK and GStreamer reliably, we need to create and
check context capabilities ahead of time. We also need to be able to query context at any
time during playback. Also we should avoid changing importers on context queries, this can
lead to all sort of problems.

To be able to achieve all of above, introduce new context handler base class that works
separately of importers.

lib/gst/plugin/meson.build

@@ -41,6 +41,7 @@ gst_clapper_plugin_sources = [
   'gstclapperpaintable.c',
   'gstgtkutils.c',
   'gstplugin.c',
+  'gstclappercontexthandler.c',
   'gstclapperimporter.c',
   'gstclapperimporterloader.c',
 ]