properly (and quickly) check for existence of required keys
figure out what an app can open from its exec key
validate the %fields in an app's exec key
localized names still don't work.
most of the app-specific stuff isn't done yet.
categories aren't handled yet (to only show in/not show in some category)
Move the .desktop file parsing details into obt/ddparse.c, which generates
some hash tables holding the data of the file.
Add a new obt/link.c which will build an ObtLink from a parsed .desktop file,
and may support other ways to create these links in the unforseeable future
