Add the GeanyCtags plugin

Hi Frank, is it OK to merge this new plugin? It adds support for generating a tag file using ctags for all project files and then querying the ctags file without any tag regeneration. This is mostly useful for really big projects which for which it might be too slow to regenerate tags every time some source is modified (one guy was interested in it at Prague Linux Days and I had this privately from the past).

Please have a look if I have provided everything what is needed by the plugin project - both autoconf and waf build works, I've created some documentation and other files present in other plugins.

(I have yet to ask Enrico if it works on Windows.)
This is deprecated in favor of `plugin_set_key_group()`.