jscam: improved camMarkTiles and eventArea<>.

camMarkTiles now accepts a list of areas to mark. Also,
it adds areas to mark, rather than replaces them.
Add new function: camUnmarkTiles.

New decorator: camAreaEvent, to simplify eventArea coding.

Update the levels to use new features. Areas are no longer
triggered by scavenger units, but only by player units.

refs ticket:4234