pidgin/pidgin

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

Testing Done:
Compiled

Reviewed at https://reviews.imfreedom.org/r/461/
REVIEWBOARD_URL = "https://reviews.imfreedom.org/"
REPOSITORY = "pidgin/pidgin"
REPOSITORY_TYPE = "mercurial"
BRANCH = "default"
LAND_DEST_BRANCH = "default"
LAND_PUSH = False