- various engine updates from Raze.

* removed refreshFreq variable and related code. This only got into the backend because of stupid interpolation code in some of the Build games which has long been removed.
* save FixedBitArrays as base64 in savegames.
* allow indirections in the string table - by prefixing the language string with '$$' the remaining text is interpreted as another string label to resolve.
* constexpr in palette code, also replacing #defines with enums.

src/common/engine/i_interface.cpp

@@ -2,7 +2,6 @@
 
 // Some global engine variables taken out of the backend code.
 SystemCallbacks *sysCallbacks;
-double refreshfreq;
 FString endoomName;
 bool batchrun;
 float menuBlurAmount;