Please tag releases (needed for downstream packagers)

[danfe]

When maintaining GtkRadiant packages in various GNU/Linux and BSD distributions, it is often desirable to be able to coalesce each released version to a particular commit. This is currently missing here (only 1.5-release from 17 Mar 2012 is tagged). Could you please pick the right commit which corresponds to 1.6.6 and tag it? Naturally, this should be done for the past and future versions as well. This had been asked before in #334 and #120.

P.S. Issues are supposed to be closed after they were fixed, not before.