Allow SLOT CELL PLAY MODE to be changed without triggering cell

[dotherightthing]

This control is currently used as follows:

1. Select a slot
2. Set Grid Mode to Button
3. Click a grid cell to start playing it
4. Click SLOT CELL PLAY MODE to change cell sample play mode

or

1. Set Grid Mode to Button
2. Click a grid cell to start playing it
3. Select a slot
4. Click SLOT CELL PLAY MODE to change cell sample play mode