Make titlebar on MetroWindow non-focusable

By default the first "tab" pressed focuses the titlebar of a MetroWindow, as shown here:

![tabfocus](https://cloud.githubusercontent.com/assets/548524/2717247/7406cd58-c53c-11e3-8432-9d4ea84f904c.png)

This pull request adds `Focusable="False"` to the `ContentControl` responsible for this to stop it happening.