Add more exports to StumpWM API

As we know, the "StumpWM" Common Lisp package exports a lot less functions than people actually use in configurations/modules.

I tried to add some missing exports and moved one missing function from windowtags module to wse (WSE uses the function).

The changes are mostly local; mass changes are:

 Window slot accessors are made public. Rationale: _changing_ the set of window slots is done by a public function; modules need window attributes constantly.

```defcommand``` exports the created command. Rationale: exporting to UI seems to be an even more public operation than exporting to the API.

Currently I can load my configuration (couldn't do it before).