Fix backspace on entry field problem on OSX

2 problems with backspace on OSX on an entry field:
- Pressing backspace on an entry inserted a character; `KeyTypedEvent.Glyph` contains a character on backspace.
- Once the previous problem was fixed: Focusing on an entry by clicking sets the `selecting` field to true, which caused problems with the range calculation on backspace and delete.

(This is my first time contributing to an open-source project, so please be gentle :smile:)