Show horizontal scroll bar when scrambled letters are too long

[svlandeg]

Fixes #66

This isn't entirely yet how I wanted it, because:

• on desktop, the bar only shows up after hovering with the mouse in the correct place
• on mobile, the bar shows only after scrolling with your fingers in the correct area, but then it shows on top of the letters.

But I couldn't find a better solution yet to suit both platforms, so it'll have to do for now, and at least it should be functional & more usable than before.