Spell Hotkeys F5-F8 on Mobile

[OlafForkbeard]

So this thing is awesome.

However Casters are very difficult to play without hotkeys. At the beginning of Caves you will often need to rapidly switch between Fire and Lightning spells. On desktop you can just use the hotkey. On mobile however the fastest way is to leave your spellbook open.

I really like how the belt is along the side of the screen, but I can't help but think that some real estate could be F5-F8. All 4 take too much space, but a single button could cycle through them.

I'm not even sure how to impliment it, as it's beyond my knowledge scope. So here is to hoping someone can assist me.

[patricksommer]

I just opened a PR for this. Please consider the changes! @d07RiV