check-headers: give a clue why the copyright might be invalid

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix style of license headers

check-style wants an empty line between license headers and package
statement.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add Javadoc for DefaultLegacyEditor

Thanks, July Jenkins, for pointing it out.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>