Addresses #2561 by setting the charCode on keydown events only when the keycode is for an enter, return or tab key without modifiers.

This prevents lastChar from being set incorrectly on compound key events. E.g., lastChar is now not set to 'V' on a command-V keydown event.
Merge pull request #2562 from iwehrman/iwehrman/implicit-hints-on-enter-and-tab

Implicit hint requests only on enter and tab keydown events