The incredible editable textual egg.

Refactor TextPad and Textbox editable text gadgets. Added public replace_text methods for both which replaces private _del_text and _add_text methods and simplifies undo/redo stack handling. Contiguous text edits are now joined in undo/redo stacks to save space.

Continue to try to fix dtype size issues on different systems, revisiting #39 once again.