Only warn about duplicate menu entries if priorities are equal

When we try to add menu items with the same path and their priorities
differ, we should not complain (the higher-priority one was already
added in that case, so we do not have to replace any existing item).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #77 from scijava/suppress-warning

Only warn about duplicate menu entries if priorities are equal