HTML support

[blockspacer]

First attempt: I managed to add litehtml support to imgui, but without support for complex CSS. Inspired by #973 (comment) see for full code litehtml branch https://github.com/blockspacer/qtimgui-wasm-port/blob/feature/litehtml/container_qt5_imgui.cpp

Second attempt: Now i ask for any help with a lot better HTML/CSS system, with animations e.t.c. https://github.com/blockspacer/skia-opengl-emscripten

I found cobalt.foo project - tiny, but powerful HTML/CSS engine. I removed JS engine from it, so it became small and can run in browser (small browser in browser!). For now that cobalt-for-browser port can create custom HTML nodes and bind C++ callbacks via HTML attrs (like onclick="dosmth").

Just wrap your GUI element into html node and bind cobalt event to your GUI event. See blockspacer/skia-opengl-emscripten#3 for details