Mouse wheel

The mouse wheel is used to manipulate the size of objects. What can be sized?
 * Move the mouse wheel forward to enlarge the object.
 * Move the mouse wheel backwards to reduce the size of the objext.
 * The model as awhole: 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
 * EEG sensors: If the mouse cursor is over an 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.