Added Textfield class

I added a full ```PretzelTextfield``` class! Well, not full, but it has all the basic functionality of editing strings and saving / loading them correctly, in a similar way to the textfield in Params.

This is what's not implemented yet (also listed in _Textfield.h_):
 - make textfield clip and scroll text
 - accept int and float variables
 - allow to move cursor position with mouse or arrow keys
 - allow copypaste
 - lose focus after not editing for a while?
 - fire signals on start and end editing, like iOS

I also updated the example to show how to use it.