core: Check if plugin is already loaded
core: Check if plugin is already loaded

When loading a plugin, check if the plugin is already loaded. If so then skip
it and return an error.

Signed-off-by: Juan A. Suarez Romero <af79b5692270b306571bb1d8bbe0285ae3ac93d9@igalia.com>

kyoushuu/grilo,grilofw/grilo,MathieuDuponchelle/grilo,kyoushuu/grilo,MathieuDuponchelle/grilo,GNOME/grilo,grilofw/grilo,GNOME/grilo,jasuarez/grilo,jasuarez/grilo,kyoushuu/grilo,jasuarez/grilo,MathieuDuponchelle/grilo,kyoushuu/grilo,grilofw/grilo