BrowserInternalElement.Focusable removed, instead used converter
Merge branch 'Sitrus2' into DUI-ReplaceBrowserItem.Focusable
Empty string removed.
DataContextToBool renamed to DataContextToBoolConverter.
And key also renamed.
Merge pull request #72 from vmoyseenko/DUI-ReplaceBrowserItem.Focusable

DUI: Replace BrowserItem.Focusable property