Unified tag icon handling while adding support for different icons per monitor.
In general LENGTH(tags) has been replaced with a NUMTAGS macro (defaulting to 9)
and the tags[] array has been replaced with a tagicons[][] array, access to which
is done through a single function tagicon.
This allows one central place where alternative tags, alttagsdecoration, or other
future tags logic is handled. This also gives a consistent display of tags
regardless of the module that presents tags.
Additionally the monitor index has been integrated into dwm for easier access.
