Create ImageJ2Options class

This options plugin will be used to enable or disable IJ2 features in
the IJ1 gui.
LegacyInitializer: display UI

After initialization, we need to tell the IJ2 UI to display. If we're in
legacy mode, this will trigger the LegacyUI to "display" and
corresponding reactive events to trigger (such as displaying the Welcome
to IJ2 window).
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.
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.