Spectacle's Desktop file assumes that qdbus is in $PATH. However this is not
guaranteed; the command lives in the Qt binaries dir which is not typically in $PATH,
and distros typically create symlinks with different names in /usr/bin, but not all
create a second compatibility symlink at /usr/bin/qdbus that points to the version
for the current Qt version.
Therefore, we should find the command in the Qt binaries dir itself rather than relying
on these compatibility symlinks.