Handle emoji correctly when deleting text

[gitterrost4]

We expand last-character-detection to handle emoji (especially those connected by zero-width-joiners such as families with different skin tones) correctly when hitting the backspace key.

Fixes #1168

[dessalines]

Could you link where you got this from? It's kind of weird to me to have so much custom code with exceptions for a delete key.

[gitterrost4]

I basically just read up on how to handle unicode sequences on android:

https://developer.android.com/reference/android/icu/text/BreakIterator

Also here is a cautionary sentence in the documentation of deleteSurroundingText and right below it is the documentation of deleteSurroundingTextInCodePoints, which would make the code a lot simpler, but is only available from API 24:

https://developer.android.com/reference/android/view/inputmethod/InputConnection#deleteSurroundingText(int,%20int)

[dessalines]

Deleting a key is such a simple action, that I'd rather see how other keyboards are doing it before custom-coding something from scratch.

[gitterrost4]

I looked at florisboard in detail, but it's a lot more involved there.

[dessalines]

Looks like AnySoftKey sends delete events: https://github.com/AnySoftKeyboard/AnySoftKeyboard/blob/d1edc81ede3bc946740462ae40d33774e8bd3d95/ime/app/src/main/java/com/anysoftkeyboard/AnySoftKeyboard.java#L891

I'm still not sure whether we should be doing these complicated hacks to do something as simple as deleting a key, just because a few applications improperly hijack key events. I'll merge for now, but if this causes more problems and more complication down the road, then I'm going to switch back to a simple delete keyevent, and ask people to open up issues on those applications.

[gitterrost4]

Anysoft also only sends the del key in specific circumstances. If you look at line 913, it also does some calculation with code points.