ui: add common textbox keybindings

Here are some:
* ctrl+a moves to beginning of the line (`Dir(math.MinInt16, 0)` as the X-axis counterpart to Home)
* ctrl+e moves to the end of the line (`Dir(math.MaxInt16, 0)` as the X-axis counterpart to End)
* opt+left moves to the beginning of the current word (`Dir(-2, 0)`)
* opt+right moves to the end of the current word (`Dir(+2, 0)`)
* ctrl+w deletes the previous word
* ctrl+u deletes to the beginning of line
* ctrl+c should copy
* ctrl+x should cut
* ctrl+v should paste
* [more?]

(On Linux Opt==Ctrl?)