clean up PidginCellRendererExpander

Cleanup the PidginCellRendererExpander

Rename gtkcellrendererexpander.[ch] to pidgincellrendererexpander.[ch]

Reimplement the activate method so that the user clicking on the expander will toggle it.

Testing Done:
Connected a bonjour account, expanded buddies and groups. Also ran `ninja pidgin-pot`.

Reviewed at https://reviews.imfreedom.org/r/197/