Added alttagsdecoration patch.

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.

patch/bar_tagicons.h

@@ -0,0 +1,7 @@
+enum {
+	DEFAULT_TAGS,
+	ALTERNATIVE_TAGS,
+	ALT_TAGS_DECORATION,
+};
+
+static char * tagicon(Monitor *m, int tag);