Include functional.
ResStr can now notify listeners on reload.
Refactor Ttf::RenderText() into FmtTextDecor.

This way, we only recalculate the formatting when the text changes, and
in the future we will be able to store info about things like clickable
regions, etc.

Ttf::RenderText() will remain for now, since it's a convenient and fast
way to render arbitrary formatted text.