NumericUpDown' value min-max validation issue

Value is not validated when changing it manually. For example if 999 is typed by the user
and maximum value is set to 100 then view model value will be equal 999 instead of expected 100. At the same time NumericUpDown value is equal 100 because it is coerced in the CoerceValueCallback.

In order to understand what I'm trying to say I added NumericUpDownValue property to view model and a Label with content binded to this value in the commit [e38ff2b](https://github.com/alanissoftware/MahApps.Metro/commit/e38ff2b08393b5d27a80de724a2a780b5c860756). Commit [b7efd17](https://github.com/alanissoftware/MahApps.Metro/commit/b7efd17de57720b832c66ed4eb32f051426a2806) contains the fix to this issue.