Simplify service population for commands

Now that Context#inject(Object) populates service parameters, we can
reuse that feature rather than having a special case for Commands.

We do then need to have a special exclusion for such parameters in the
CommandInfo wrapper, though.
DefaultRecentFileService: handle missing command

If the FileOpen command is missing, we should handle it gracefully via
defensive programming. This avoids a unit test failure in
imagej.script.JavascriptTest.