[LLDB][GUI] Add submit form key combination

This patch adds a new key ALt+Enter key combination to form windows.
Once invoked, the first action is executed without having to navigate to
its button.

Field exit callbacks are now also invoked on validation to support this
aforementioned key combination.

One concern for this key combination is its potential use by the window
manager of the host. I am not sure if this will be a problem, but it is
worth putting in consideration.

Differential Revision: https://reviews.llvm.org/D108410