Automatically focus the dialogs when they show up

This commit fixes the automatic focusing for arbitrary dialogs and
enables the focusing for advanced message dialogs.

We also enable the focus border for the dialog buttons so the currently
focused button is actually indicated.

Fixes #1679
Updated the v1.0 release notes
Merge pull request #1684 from MahApps/fix-dialog-focus

Automatically focus the dialogs when they show up