Add modal windowing

Adds style to Shoes.app calls for making things modal.

Also supports a #wait_until_closed so we can hold until we get an answer
for model dialogs.
Shoes-based implementation of ask dialog
Don't need that old SWT ask anymore
Merge pull request #939 from shoes/shoes_ask

Better input sizing on ask dialog

* Changelog: improvement
* Replaces the SWT implementation with a shoes implementation, better reusability but might give us headaches down the road through multi app, let's see :)