Add microui sample including the original demo (without icon rendering)

[Skytrias]

#3

not sure about the license part and how we would deal with outside sources.

haven't tested it on windows

here is a side to side comparison to sgl-microui

[Skytrias]

test

[martinfouilleul]

not sure about the license part and how we would deal with outside sources.

Could you put microui.c, microui.h and LICENSE in a src/microui subfolder (so that the external code is clearly identified with its own license)? Thanks!

[martinfouilleul]

Thanks!