Selection versus highlighting

Selection and highlighting are ways to identify and prepare nobs for a command to be applied to them. However, they differ in several ways:
 * Only one nob can be highlighted at a time. Although it is possible to have only one knob selected, the purpose of a selection is to mark several knobs.
 * Highlighting happens automatically as soon as the mouse cursor is ver a knob. Selection has to be explicitely caused by the user.
 * The highlighting of a nob automatically disappears if the mose cursor is no longer over the knob. A selection stays active until the user explicitely stops it.