Handle WM_CLASS changes

Handle WM_CLASS changes

http://tronche.com/gui/x/icccm/sec-4.html

> The WM_CLASS property (of type STRING without control characters)
> contains two consecutive null-terminated strings. These specify the
> Instance and Class names to be used by both the client and the window
> manager for looking up resources for the application or as identifying
> information.

i3 processes changes to WM_CLASS by updating the cached property and
running assignments. This allows the property to be used in criteria
selection.

fixes #1052