Add font selection input to events and adds a set font event

[Mico27]

This adds font selection input for custom events and adds a "set font" event that makes uses of the VM_SET_FONT gbvm operation.
( This PR was made is mainly because I want to be able to use the font selection input for my plugin )

[chrismaltby]

Thanks @Mico27

Made a few small changes so the generated GBVM will use symbols which are in game_globals.i to make it slightly more readable:

        ; Set Font
        VM_SET_FONT             FONT_GBS_VARIABLE_WIDTH

and I updated setFont to take in an id string rather than looking up the index and passing it in.

The idea is that

• ScriptBuilderBase functions should deal with low level things generating the raw GBVM and mostly taking symbols/indexes/constant values as inputs. Functions here are subject to change a fair bit as the engine is modified.
• ScriptBuilder functions should be more high level, dealing with IDE constructs like the string ids stored in gbsres files. They should provide a reasonably stable wrapper around the internal functions from ScriptBuilderBase (which should mostly be prefixed with _). As the internals of the engine change I try to keep these up to date to call the right internal functions to keep logic consistent between GBS versions.
• compile function in events, ideally only use functions from ScriptBuilder and don't get involved with the internals. That way they shouldn't need updating if the GBVM operations change. I know this isn't the case for many plugins that need to get involved with internals but for the inbuilt events at least I prefer to use the "public" functions from ScriptBuilder instead (though there's a few of the more complex events break this rule like "eventReplaceTileXYSequence" I'd prefer to avoid internal functions in events where possible)

^ I should probably document this somewhere :-)

Thanks,
Chris