GDesktopAppInfo: avoid inotify on missing dirs

Some desktop file directories, like /usr/local/share/applications may be
missing on some systems.

When we try to inotify on these directories, this will result in a
every-4-seconds poll being setup which is quite bad.

This is an issue that should be fixed in inotify itself but the problem
is much larger there.  For now, we can work around it in GDesktopAppInfo
by refusing to monitor missing directories.

We may get some spurious notifications of changes in the case that
/usr/local/share or /usr/local/share/applications is created without
actually adding desktop files, but spurious changes can already be
reported in other cases, so that's OK.  We won't get (user-visible)
notification for a simple case of a completely unrelated file being
created (however we cannot avoid the wakeup in this case due to how
inotify works).  That's probably pretty theoretical, though, since files
in /usr don't change much and for the home directory we're likely to
have at least ~/.config and ~/.local existing.

https://bugzilla.gnome.org/show_bug.cgi?id=736350