ij-legacy: clarify why we patch MacAdapter only sometimes

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add an application event for opening files

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MacOSX: handle event when files are dropped onto the application icon

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ij-legacy: do not let MacAdapter do its thing

If we let MacAdapter run normally, we will not be able to install any
other application listener. But ImageJ2 needs to.

For the moment, this breaks Fiji's legacy mode again: neither Quit nor
Drag 'n Drop of files onto the Dock icon will work. We will fix that in
the next commits.

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