Inspired by #504.
Instead of requiring the user to do PLATFORM_ANDROID #ifdefery,
have the android_main entry point exported by raylib and call
the user-defined main. This way many games could (in theory)
run unmodified on Android and elsewhere.

This is untested!

- Removed useless templates
- Reviewed all Makefiles
- Rework some code for Android support
- Added resources to advance template

Some basic to advance templates are provided to be use as base code for
new games