Added returns, fixed add_filepool_control (v2)

Added returns and improved add_filepool_control, which should now work either with or without a custom function.
Added set_pos and fixed get_pressed

set_pos now with proper doc
get_pressed now calls pygame.event.get() (otherwise pygame.mouse.get_pressed() doesn't quite work)
Added set_pos and get_pressed to psycho mouse

Now including the runtime error when set_pos is called in a pyglet environment

Slight point of comment: now only check first window in existence, which is fine for now, but could be reconsidered if support for multiple monitors is ever added. But, now that I think of it: why would one create one pygame psychopy window and one pyglet?
Merge remote-tracking branch 'esdalmaijer/patch-9' into playground
Merge remote-tracking branch 'esdalmaijer/patch-10' into playground
Prepare plug-in infrastructure for qtautoplugin
Do not require a return value for `item.prepare()` in sequence item
Merge remote-tracking branch 'esdalmaijer/patch-7' into playground
Clean-up code and comments in qtplugin.