purple: build against GPlugin-based 3.x API
Pidgin 3.x changed the plugin API substantially. Instead of filling
PurplePluginProtocolInfo structure with function pointers, plugin
developer is now supposed to inherit PurpleProtocol GObject class and
implement its various interfaces.
A file which implements the new API, purple-plugin3.c, was added to
Sipe Purple backend, whereas the legacy code for libpurple 2.x.x
remained in purple-plugin.c. Depending on libpurple version for which
Sipe build was configured, the corresponding file is picked.