Moved VST sync functionality into new VstSyncController class

First attempt to clean up the mess in the Song class by moving the VST sync
functionality into its own class and just calling a few functions from it.
VstSyncController: print a warning if VST sync support is not enabled

Users might wonder why VST sync does not work as they didn't notice the
option in the configuration dialog.
Song: removed old action array

As of commit c60e7ba8d1a589e66f4bb9ef4c09726b1d6c594f we no longer queue
playback actions and process them later, therefore remove old remains.
Song: update VstSyncController in new playback control methods

As of commit c60e7ba8d1a589e66f4bb9ef4c09726b1d6c594f the whole playback
control has been revised. As this was an old patch, it didn't take care
of the VST sync functionality. This is fixed now for the most part.

Closes #184.