Keystrokes stuck when typing into MathQuill using IME (Input Method Editor)

[64qq]

Users of languages with non-ascii characters type in both alphanumeric input mode and IME mode. If a user accidentally types in MathQuill with the IME on, incomplete entries are made and get stuck, and sometimes the backspace and arrow keys also stop working.

These are not actually stuck; they are typed in the hidden textarea, but the user cannot see that they are in a composition session, so the keys look as if they don't work.

It's preferable to block text input from the IME, as it's not needed to type formulas.

[drlippman]

I just wanted to add a note for anyone else who runs into this: For some reason, on Chrome in Android every character (even basic alphanumeric ones) gets sent as IME. The changes in #289 block anything that appears to be from IME input, preventing the on-screen-keyboard from being usable for any input. Reverting the change in #289 worked for me to restore basic typing ability.