There was a bug introduced by LMMS#5657 where reloading a project
and playing it could cause a Segmentation Fault crash. After some
debugging, @DomClark tracked the issue to be likely a use-after-free
being caused by m_oldAutomatedValues not being cleared when the project
was loaded again.
	This commit adds a line to clear the m_oldAutomatedValues map on
Song::clearProject(), which is called from Song::loadProject().

Co-authored-by: Dominic Clark <mrdomclark@gmail.com>

	Now, instead of using a Signal/Slot connection to move the
control of the models back to the controllers, every time the song is
processing the automations, the control of the models that were
processed in the last cycle are moved back to the controller. The same
is done under Song::stop(), so the last cycle models control is moved
back to the controller.
	That removes the need to have a pointer to the controlled model
in the controller object.

	Adds mixer model change request to avoid race condition.
	Revert changes to processAutomations.

	Since we are iterating from QVector::begin() to QVector::end()
there's no need to check if it's empty first.