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.
sf2_player: fix #481
Dual Filter fix: make more efficient -  don't calculate coefficients unnecessarily
Change Linux Multimedia etc. to LMMS in desktop/package files
Merge pull request #487 from diizy/stable-0.4

Dual Filter fix: make more efficient -  don't calculate coefficients unn...
Merge pull request #484 from grejppi/stable-0.4-sf2velocity

Fix for #481
Merge https://github.com/LMMS/lmms into bug468
basic_filters.h: Tweaked limits for cutoff.