PixmapButton - Undo/Redo

Undo/redo of PixmapButton shows erratic behavior. A single press of a PixmapButton seem to indicate that undo/redo isn't implemented for this class but consecutive presses will register but it's glitchy.

Example one:
Press mute button on a track and then press or use the edit menu function. The action isn't reverted.

Example two:
Press mute button on a track twice followed by two 'undo' actions. The second undo will change the state of the button.

Example three:
With two unmuted tracks in the song editor do the following.
Press the mute button on the first track four times.
Press the mute button on the second track four times.
Press once. Nothing happens.
Press three times. The second track will change state to muted.
Press once. Nothing happens.
Press three times. The first track will change state to muted.

Start by reproducing the three examples in the LMMS song editor, using the PixmapButton mute control, Ctrl+z, and the Edit menu. Trace the PixmapButton action into the undo/redo handling; done means each described sequence reverses the expected mute changes in the correct order.

Written by the indexing model from the issue text.