Implement an idea for a start screen.

This is just an idea so far.

Implemented a "start screen" for RGUI. Idea is let users know the default keybinds at least for PC where this seems to be a problem. The default keybinds aren't really documented anywhere, and users don't read READMEs <_<.

Currently looks like this:
![idea](https://f.cloud.github.com/assets/163683/1269237/fa65f774-2cf4-11e3-83ef-3cd041449571.png)

This obviously only makes sense for PC with the current wording, so not sure how we'd go about doing it.
It's maybe possible to create some very basic help system this way too, but that's probably going overboard. If we can link to external documentation, that'd probably be best.

You get the start screen every time you start up RetroArch, so I guess it should set some flag in the config.