Merge branch 'fiji-compat' into test-merge

This branch completes the first round of reconciliation between Fiji's
and ImageJ2's legacy patching.

The purpose is two-pronged:

1) we want to run Fiji in legacy mode without custom Fiji to support the
legacy mode (apart from setting the application title, specifying which
editor to use instead of ImageJ 1.x' one, etc)

2) we want to be able to call Help>Switch to Modern Made to switch away
from legacy into modern mode in Fiji, even after starting up in legacy
mode, and even after opening a couple of images there (which should then
be opened in modern mode, too).

This branch also opens the door for a seamless transition between Fiji
starting in legacy mode and (in a somewhat distant future) in modern
mode: the plan is to support obtaining a SciJava context (containing all
of ImageJ) by:

	context = (Context)IJ.runPlugIn(Context.class.getName(), null);

After a bit more work to speed up initialization of the context, we will
be able to provide the context with the IJ.runPlugIn() method already
upon startup of Fiji, not only after switching to Modern Mode for the
first time.

The intention of all this, of course, is to let legacy plugins
transition incrementally, first by using the services available in the
context, later (when the legacy service learnt to synchronize data also
in legacy mode while keeping the modern UI dormant) even by allowing to
run ImageJ2 plugins in legacy mode, on legacy images (insofar the data
types and image modalities can be finagled into ImageJ 1.x' data model).

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