Teach write-headless-jar a full and/or non-headless mode

It is really handy to have an ij1-patched drop-in replacement for ij.jar
in case you cannot modify the startup procedure as the ImageJ launcher
does. This change allows you to write that with

	write-headless.jar --no-headless --full-jar patched.jar

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix FijiTools.isDeveloper()

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Let FijiTools#getFileExtension hand off to scijava-common

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid overriding the legacy initializer

A much better way is to let LegacyInitializer do its job and declare
a service (that we might want to use for more Fiji-specific stuff
later) that the SciJava Context will discover and initialize.

That way, Fiji gets handled properly even when started from a patched
ij.jar written out by "write-headless-jar --no-headless --full-jar"
(because we no longer have to override things before starting the
LegacyEnvironment, or for that matter, patched ij.ImageJ).

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