Cleanup guibase/command distinction

Guibase, which still needs to be renamed, is a set of basic functions
that should be available for all Gui objects (think taglist.py and
text.py/reader.py classes). It makes sense for bind mechanics and other
functions that don't have a story / tag context to live here.

CommandHandler is now mostly vestigial but I think converting raw keys
into command strings is still separate enough to exist here instead of
GuiBase.