Fix TextArea with enter keyboard shortcut (#12424)

When a keyboardshortcut has been added to anywhere on the page,
the previous behaviour would cause the keyboardshortcut event to
be processeed before the newline was processed. The end result
was that newlines were never added when typing in the TextArea.

Keyboard shortcuts operate on KeyDown events. By adding a listener
for these events and stopping their propagation when the ENTER key
is pressed, this unwanted behaviour can be averted, and the user
can enter multi-line text in a TextArea even when Enter is used as
a keyboard shortcut.

Obviously, this means that the keyboard shortcut will not work as
long as the TextArea widget has focus. This is the new intended
behaviour.

Change-Id: Ied438acb8589df498e5634271e486517bf6ac41e