Add agi::ProgressSink and agi::BackgroundRunner interfaces to libaegisub
which represent a generic progress sink and a thing which calls funtions
that need progress sinks. Make DialogProgress implement
agi::BackgroundRunner, invoking the passed function on a worker thread
and giving it a progress sink to update the dialog with.
Rewrite Automation4::ProgressSink, LuaThreadedCall and all related
classes to be based on agi::ProgressSink. Automation now simply uses
DialogProgress (although that's merely an implementation detail) and
adds a single method to route dialog opening from the worker thread to
the GUI thread.
Originally committed to SVN as r5634.
