Add backspace to emoji drawer (+ two tiny tweaks)

Thought this would be useful, and someone was even generous enough to leave unused drawables for the purpose:

![Screenshot](https://cloud.githubusercontent.com/assets/124223/3757758/0d3b21ec-183d-11e4-9774-7b06a061076b.png)

Also two other small tweaks. (I personally think it's better without the gray placeholder squares, but that's just my opinion, man.)
Comment is probably unnecessary, this is common practice with EditTexts, and should we make this static and move it to the top of the class?