Adds a multiplayer tab to allow hosting and joining games from within the GZDoom launcher rather than needed to use the command line. This has its own set of defaults independent from the main play page which necessitated rewriting how this information is passed and stored in the backend. A startup info struct is now passed back which has its defaults set from the cvars and then propagates any changes to it back to the defaults after selection is complete, making it much simpler to interface with the engine defaults.
* 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.
