clapper: Support libpeas based plugin system

Allow loading external plugins called "Enhancers" that as the name suggests,
enhance Clapper library capabilities.

Currently implemented is a "ClapperExtractable" interface meant to extract
an actual media that GStreamer can later play from an initial input URI.
Additionally, an internal GStreamer elements that work with it are ported/moved
here from "gtuber" library that this functionality replaces.

meson_options.txt

@@ -35,6 +35,13 @@ option('doc',
   description: 'Build documentation'
 )
 
+# Functionalities
+option('enhancers-loader',
+  type: 'feature',
+  value: 'enabled',
+  description: 'Ability to load libpeas based plugins that enhance capabilities'
+)
+
 # Features
 option('discoverer',
   type: 'feature',