Merge branch 'master' of https://github.com/rheit/zdoom

+jpalomo's A_SetSpeed patch.
+Fixed: Since the serializing code cannot distinguish between user
variables of the same name coming from different actors in an
inheritance chain, this kind of name duplication must be prohibited.
+Fixed: When the SectorAction's TriggerAction method was renamed, the
inherited one in the MusicChanger was overlooked. (Bit thanks to the C++
specs for allowing to override a non-virtual method with a virtual one
without even showing a warning...)