Update windowtags module

[PuercoPop]

It references functions that have have been removed in favor the SBCL specific version

• string-to-utf8 -> sb-ext:octets-to-string
• utf8-to-string -> sb-ext:string-to-octets

An Additional complication is that the removed functions weren't as strict as the SBCL version about the type of the arguments received. SBCL insists in received a VECTOR of (UNSIGNED-BYTE 8)s.

Also it needs to import STUMPWM:FIND-GROUP

[PuercoPop]

Done in #119