Create a generic GUI helper that absorbs most of the content in all the main functions

[dfridovi]

Ideally, the user should only have to write a very simple/short main.cpp.

[dfridovi]

@chihyuanchiu This would be a really nice "warmup" GUI task - basically the idea would be to factor as much of the code in any example's main.cpp into a separate function implemented in some other .cpp file which could be just called from main.cpp so that it gets much simpler and shorter and easy to parse.