Fix the "Fiji quits twice" bug ;-)

The about, prefs and quit app events were being handled twice: once by
the AppEventService (specifically: by ImageJ2's CoreAppEventService),
and once by the ImageJ legacy layer (specifically: IJ1Helper's
LegacyEventDelegator). This duplication led to those three app events
being handled in two different ways simultaneously. Most problematic of
these was quit, which would pop two dialogs simultaneously on OS X when
Cmd+Q was pressed: one that said "Quit ImageJ?" with OK/Cancel (due to
the ImageJ2 QuitProgram command being run), and another set of dialogs
popping due to the legacy layer's quit sequence running concurrently.
Requesting an About dialog was also interesting: it popped both the
classic ImageJ 1.x dialog, as well as the new ImageJ2 about screen.

This commit introduces a LegacyAppEventService that overrides the normal
AppEventService behavior, such that _only_ the legacy layer takes care
of these three operations, in a way compatible with ImageJ 1.x.

Note that for the bug to be _fully_ fixed, a version of SciJava Common
is also needed containing the following bug-fix:

https://github.com/scijava/scijava-common/commit/6b1fd5fc3c7f00f8d443d2447f72b016caf378f1

Without that, the two AppEventServices present in the application
context (CoreAppEventService and LegacyAppEventService) will each
receive the relevant App*Events, and hence each try to handle them in
their own way, resulting in ImageJ still quitting twice, etc.