Formalize/design/unit-test IDE-support API for macros.

[nadako]

At the moment nobody really understands how to add support for completion, go-to-definition and other IDE services in a macro. There are things like EDisplay and Context.containsDisplayPosition, but currently they more or less feel like just leaked internals without any documentation or reliably specified behaviour.

For example, I just added some go-to-definition support to tink_hxx, but I have no idea if this is really correct or whether it will break with one of the next haxe updates.

This makes creating and, more importantly, using macro-powered DSLs rather painful and I think we could improve the situation, so I'm creating this issue in hope to get some discussion going and gather ideas.

Some initial thoughts:

• EDisplay node generally makes sense to me, although I think it could contain exact cursor position. This seems useful in cases when we have EDisplay(EString("some unknown DSL")) and we want to know exactly where to insert a new EDisplay from a macro when generating expressions.
• Maybe we should have macro API to directly "raise" display results instead of adding EDisplay values to be processed by the typer.
• For DSLs we often want to generate custom syntax (e.g. <tagname attr="value"> in HXX), we need some way for a macro to return a list of those, but I'm not sure how structured should it be.
• What's EDisplayNew? Seems like a leftover that we should remove to avoid confusion.

[nadako]

Some notable DSLs to consider:

• tink_hxx
• hxsl
• feel free to add more