[Mac] Fix LinkLabel
[Mac] DialogBackend: don't add padding for the button box if there are no buttons
[Mac] Fix keyboard tabbing

Set all windows we create to automatically manage the key view loop,
since there's no support in xwt (yet) to explicitly manage the tab order.
Prevent Labels from participating in the tab order
[Mac] DialogBackend: End the modal loop if the dialog is closed with the close button
[Mac] Fix setting Font on widgets that are wrapped in CustomAlignedContainer