Various build fixes:
* Added bitbucket-pipelines.yml for CI
* Removed old plugins/win32/transparency from build
* Added updated plugins/transparency.c for translation
/** @page gtkblist-signals GtkBlist Signals
@signals
@signal gtkblist-hiding
@signal gtkblist-unhiding
@signal gtkblist-created
@signal drawing-tooltip
@signal drawing-buddy
@endsignals
@see gtkblist.h
<hr>
@signaldef gtkblist-hiding
@signalproto
void (*gtkblist-hiding)(PurpleBuddyList *blist);
@endsignalproto
@signaldesc
Emitted when the buddy list is about to be hidden.