Filter by server version and forgotten server's CVars window.

Implemented filtering servers by server's version, related to #27.
This combobox is saved-restored across restarts.

Added forgotten bd04870 server_cvars.go, which is responsible for
showing server's CVars window.

common/const.go

@@ -12,3 +12,9 @@ package common
 const (
     URTRATOR_VERSION = "0.1.0"
 )
+
+// Self-named.
+var SUPPORTED_URT_VERSIONS []string = []string{
+    "4.2.023",
+    "4.3.1",
+}