JSON-RPC: fix detection of alpha build versions and add "tagversion" property

Thanks to @Tolriq for reporting this. The "tag" property of the "version" object returned by JSON-RPC's Application.GetProperties was broken since the unification of version handling. The first commit of this PR fixes that problem. Furthermore @Tolriq has asked to also provide the actual version/revision of the alpha/beta/rc builds. Using the string from CCompileInfo::GetSuffix() directly (which would e.g. be "ALPHA1") would break backwards compatibility of the "tag" property so I've added an additional property "tagversion". If anyone can come up with a better name, feel free to say so.