If you try to Compile a J.P. *.jar, in fact it does the same as Build. This is
noticeable e.g. due to Ales Novak's recent change to the compilation system
which means that hitting Compile on a single object in fact tries to Compile,
not Build, it. But doing so on a *.jar actually rebuilds contained files and the
JAR itself, which is not desirable. And Compile'ing a directory containing a JAR
will always rebuild it, so the Output Window will say that something happened,
when in fact everything might have been up-to-date.

Technically: JarUtils.createCompilerJob asks for CompilerCookie (instead of
.Compile or .Build as appropriate; the result could be either of these,
randomly). JarDataObject implements both cookies and does not distinguish
between them.

Ideally JarCompiler.isUpToDate would be actually implemented so that the Output
Window would give a more meaningful message. I.e. if all files constituting the
JAR were in fact up-to-date already, and the JAR itself in fact was newer than
all the files that would go into it, then nothing should be done and iUTD should
be true.