Mouse wheel

The mouse wheel is used to manipulate the size of objects.
 * If the mouse cursor is over the model window background (no object is selected), the mouse wheel can be used to zoom in or out
 * If the mouse cursor is over a EEG sensor, the mouse wheel can be used to make the sensor larger or smaller
 * Not yet implemented: If a selection is present, the mouse wheel can be used to expand or shrink the selection. The center of the expansion/shrinkage is the mouse cursor position.