UpToDate: improve detection of developer setting

The previous detection was failing when launching `net.imagej.Main` from
Eclipse—and probably in many other scenarios, too—due to the fact
that we now use release couplings by default. So
`net.imagej.updater.UpToDate.class` _will_ be located in a JAR file, even
when running from the IDE. But the `imagej.dir` property is normally not
set in that scenario, since only the ImageJ Launcher sets it by default.

Of course, you could set `imagej.dir` in your Run configuration, which
would defeat this heuristic. But this approach should work for most
people most of the time...