When inputting uncommitted text from an IME, this is now stored in a
std::string imebuffer, just like keybuffer. It also enables extended
editing events, so text longer than what fits in the standard editing
event is also supported. This commit does not yet display the text
onscreen.

This shows the uncommitted text in a box in the bottom left corner.
This doesn't show the selection (defined by the start and length fields
in the event) yet, but this is already much better than it was on its
own, and I don't know how urgent the selection is since it's broken on
Windows anyway.

Decided to implement it anyway since the broken behavior (selection
length always being 0, at least on Windows) may get fixed later in SDL,
so let's do it right in one go.