Previously the owning window's PreviewMouseDown event, used to auto close the popover, was being subscribed to
on the popover's load event and unsubscribed on unload however this didn't take into account that the Load could be called multiple
times. The popover now keeps track of if it's subscribe to the given event using a disposable handler, which is also used
to unsubscribe.

* Background and Foreground colors are now dynamically applied using the WhiteBrush and BlackBrush respectively.
* MetroPopover doesn't partake in layout (Visibility is always Collapsed).
* Popover adorner is only attached when open.

Turned out that the popover needed a logical tree like this:

MetroPopover -> MetroPopoverWindow -> Popover content

This is presumably because WPF uses the logical tree, rather than visual tree
when refreshing Dynamic Resources.

Previously clicking a dropdown item would cause the containing popover
to close if it's AutoClose is set to True (default). This occured because the
combobox uses a popup to display the dropdown which is outside the
popovers visual tree which meant the popover thought that the user had
clicked elsewhere in the window. This is fixed by using DependancyObject.GetParentObject()
which falls back to the logical tree when required.