Start up legacy service in legacy mode

While the LegacyService is being initialized, we cannot rely on any
ImageJ2 services yet. So the best idea is to say that we're in legacy
mode. If an ImageJ 1.x instance is currently running, our best bet is
to stay in legacy mode. Otherwise, let's switch to modern mode just
after initialization is complete -- toggleLegacyMode() knows that it
should not try to show anything if no UI is available.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Switch To Modern Mode: toggle all ImagePluses WindowManager knows about

When starting Fiji in ImageJ 1.x mode, switching to modern mode after
opening images should take them to modern mode, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
CodeHacker: mark classes explicitly as frozen after loading them

This hopefully helps a little bit with the upcoming move of stuff from
Fiji's IJ1Hacker to ij-legacy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
CodeHacker: commit all handled classes at the end

That should help with the upcoming move of stuff from fiji-compat:
some classes are handled multiple times and in different ways, so we
should only define the classes at the very end.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Move some backwards-compatible legacy stuff from fiji-compat to ij-legacy

This adds back some legacy methods that should have been marked as
@Deprecated instead of removed without warning.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Move fiji-compat's special handling for Apple's Mighty Mouse

On some Linuxes, the horizontal scroll wheel of the Mighty Mouse is
mistaken for a popup trigger. This change adds a work-around.

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