pidgin/pidgin

Make PurpleWhiteboard derivable and remove purple_whiteboard_[gs]et_ui data.

Testing Done:
Compiled

Reviewed at https://reviews.imfreedom.org/r/461/
[main]
host = https://www.transifex.com
[pidgin.pidgin_pot-for-development-branch]
file_filter = po/<lang>.po
source_file = po/pidgin.pot
source_lang = en