Update PianoView.cpp

Add new keys to control (Windows, Linux), replace Windows "/" key.
Disable Sfxr preview for now, possibly reimplement after 1.0.0
Merge pull request #479 from diizy/stable-0.4

Disable Sfxr preview for now, possibly reimplement after 1.0.0
Fixed FX channel of Erazzor preset

Thanks to Tres for pointing out this issue.
Merge pull request #480 from dnl-music/patch-1

Update PianoView.cpp
MainWindow: coding style improvements
Control play/pause buttons in slot of MainWindow via signal in Song class

The engine class as the component instance manager is the wrong place to
control the play/pause buttons. Instead emit a signal in the Song class
and update the buttons in a slot in MainWindow. This fixes problems with
GUI/pixmap operations happening outside the GUI thread when exporting a
project.

Closes #435.