plugin: Rework code to use new GL context handler

Create a GL context handler subclass of recently added context handler base class and
rework code to use it. This simplifies a lot of things, like: switching of importers
at runtime, handling context queries, sharing data between importers, etc.

lib/gst/plugin/meson.build

@@ -61,4 +61,5 @@ if build_gst_plugin
   )
 endif
 
+subdir('handlers')
 subdir('importers')