Improved handling of the input for TextPresenter

Also, I fixed the problem where pressing the A key doesn't insert any text.
Please, note that I omitted the "this" keyword. This is much discussed, but I have researched to form an opinion on this coding style. Anyone that needs justification, please, contact me.

Thanks :)
Removing this check means that a TextBox with AcceptsReturn = false doesn't behave correctly (it accepts returns)