This replaces all current callers of gs_file_ensure_directory with
equivalent code.
Actually, two instances were calling gs_file_ensure_directory with
FALSE, i.e. error out on EEXIST, but those cases seem fine with the
do-nothing-if-exists semantics.
A new user might think that APPDIR is the location of the app to be
built, and run something like `xdg-app-builder . ./manifest`. This
could silently the user's entire project that they are trying to
package, which is not acceptable at all! Even if you think it is their
fault for not reading the manual first!
This commit means that APPDIR is no longer deleted. Instead,
xdg-app-builder checks whether it is empty and, if it is not, it asks
the user to delete the contents and then rerun it.
This means you now have to do `rm -Rf APPDIR; xdg-app-builder APPDIR
MANIFEST` when developing your manifest, but I think that's better than
having a build tool that can optionally delete your whole project.
