IJ1Helper: make Macro.abort() work as expected

The thread name needs to end with Macro$, or else the macro
will not abort as expected when Macro.abort() is called.
IJ1MacroEngine: return null if macro was aborted

Otherwise, the display framework tries to display the
string "[aborted]" to the user, which is not expected.
Merge remote-tracking branch 'upstream/master' into ports