window: Inline contact request creation

This commit gets rid of the dialog for adding contacts. We now show the
widget for new contact requests in chat stack. While this adds some
complexity it feels cleaner than using a dialog.
request create widget: Add "valid-data" property

Will be used to enable/disable the win.send-request action.
window: Enable/disable send-request action

Only alow to activate the send-request action when the Tox ID looks
valid.