- change the line edit to accept actual text input in a form similar to
  QKeySequence::fromString (with special "native" form on OS X)
- add a button that allows entering a key sequence by pressing keys,
  including support for e.g. escape key, which was broken before because
  it closed the dialog
- add a warning label, that allows filtering the list for all
  potentially conflicting shortcuts

Task-number: QTCREATORBUG-6
Change-Id: I94fc63525f653127e87f6ef2bffe72d8dcaa867d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>