BF: Fix ReadmeFrame not showing again after close

After closing (and not just hiding) the ReadmeFrame in Builder it was
not possible to show it again using the "toggle" option in the "view"
menu. The ReadmeFrame removes its reference from parent when closing.
Using the menu entry then caused an AttributeError. This was fixed by
checking the attribute (which exists because of __init__) and calling
updateReadme() and showReadme() if it does not contain a ReadmeFrame.
ENH: Set volume in MovieStim (useful to mute sound by using volume=0)
BF: Fixed memory leak in visual.MovieStim

Two problems caused a memory leak when creating multiple MovieStim
instances across the experiment:

1.)
The line "self._player._on_eos=self._onEos" creates a reference to the
MovieStim object that prevents it from being garbage collected, that is
__del__ was never called

Solution:
set _on_eos only on playback and reset it on movie pause, stop and eos

2.)
The __del__ function did not release the memory occupied by the movie.py

Solution:
replace self._clearTextures() (which is not defined in MovieStim) by
self._player.next() (which releases the memory)
BF: movie autodraw