A switch click on one of several selected layers passed that layer alone to
`layer.setSwitch`, so Lock, 3D, Video (and the rest of the A/V Features and
Switches columns) had to be set one layer at a time.
The click now gives the switch to the whole selection, with the layers it
doesn't apply to left out (3D on a camera, Effect on a layer without effects,
Audio on a layer without audio, ...). The targets are picked when the press
happens, so drawing a row costs nothing more than before. A row that isn't
selected still changes that layer alone, pressing and dragging over rows is
unchanged, and Alt-clicking an A/V feature sets the selected layers apart.