Misc fixes

[rokups]

Sorry for moving slow, here are my promised fixes.

• Fix build on UWP. Aside from this one tiny detail RmlUI works out of the box on UWP! 🥳
• Fix including extra code using RMLUI_*_USER_EXTRA. Nobody was using it and did not notice that random code in these macros break build because pasting that code within __has_include() is not well-formed.
• Fix build with EASTL due to undeclared DataModelHandle (wanted by eastl::function).

[rokups]

Actually do not merge just yet. There will be more like DataModelHandle fix. Waiting for my CI to go green.

[Dakror]

FYI you can mark you PR as a draft until you're happy with it.

[rokups]

Huh how?

[Dakror]

On the right where you can see assigned, reviewers etc, there's a text-link based button to convert to draft, somewhere

[rokups]

Nice! Thanks 🙏🏻

[mikke89]

Thanks! I'll wait for your green light then.

I think we're probably going to keep breaking EASTL unless somebody adds it to the CI ;)

[rokups]

All good, took me few iterations where to put DataModelHandle include. No worries about breakages ;)

[mikke89]

Alright, thanks for this! Nice to know it builds on UWP as well now :)