This patch allows multiple marks to be set on a single window. The restriction that a mark may
only be on one window at a time is still upheld as this is necessary for commands like
"move window to mark" to make sense.
relates to #2014
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
