Add left/right buttons for mobile race GUI

[GustaLOLE]

What I've made for now:

• Add this buttons in right of the accelerator;
• Add a option to change to this in init_android, multitouch_settings and race_paused dialogs.

To do later: Make the buttons clickable only sliding the finger.

Video demonstrating this control: https://www.youtube.com/watch?v=Db32VbMjiJc

Some photos:

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[pedropaulosuzuki]

@GustaLOLE here's my suggestion for the Nitro icon on mobile:

[GustaLOLE]

Ok, I liked this, I hope this works (with my adjust), thx