Create ImageJ2Options class

This options plugin will be used to enable or disable IJ2 features in
the IJ1 gui.
DefaultLegacyOpener: respect useSCIFIO option

The useSCIFIO option will now turn the DefaultLegacyOpener on or off.
Add LegacyInputHarvester

We needed a trivial extension of the SwingInputHarvester that is keyed on
the LegacyUI.NAME field. We can still use the Swing harvester, in
keeping with the Swing implementation of other Legacy components.
fix: add title
fix: remove comments
fix: really remove run method
TEMP: update ij1-patcher to 0.4.4-SNAPSHOT

Replace with release version update
DefaultLegacyHooks: show LegacyUI

On creation of an ij.ImageJ instance, if we are in legacy mode, we
should "show" the LegacyUI to trigger appropriate events, since the
visibility of the LegacyUI is keyed on the ImageJ visibility.
Create IJLegacyApp

Adds a default App implementation for the imagej-legacy layer.
Add dependencies for text handlers

Adds the scijava text handlers (markup and plain text) as maven
dependencies.
Create a legacy text display viewer

Creates a new DisplayViewer for handling text in the LegacyUI. The
DisplayWindow framework is bypassed, rendering text via IJ.showMessage.
Create WelcomeToImageJ2 plugin

Adds a new plugin which checks if any ImageJ2 options are unset when
the LegacyUI is shown.

If they are, a welcome message is shown along with the ImageJ2Options
dialog.

The displayed welcome text can be modified in the WELCOME.md file.