We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
This is not only easier to comprehend code-wise and avoids some function
overloads, but also makes resizing the board while preserving game state
*a lot* easier. We now no longer have to allocate a new board on every
resize, we just grow/shrink the individual row vectors.
Also fixes a crash when clicking the board widget outside of the drawn
board area.
