[PATCH] Gamesetup map filter for new maps

[elexis]

Since we had many new maps added this and previous release, it were nice to filter those out from the prior well known ones.

The attached patch filter all maps of alpha 20 and 21 as discussed with Itms.

Notice the tags have to be updated upon release, which shouldn't be too much of a hassle if it's documented in an obvious place.

[elexis]

In 18764:

Add gamesetup filter for new maps, fixes #4246.

Don't complain if there are no maps of the selected maptype for the current mapfilter.

[elexis]

Agree with niektb that it's more consistent to only have maps of the current release listed as new, also easier to maintain.