Welcome to ImageJ2

Adds an ImageJ1 plugin for controlling ImageJ2 options (currently just disabling SCIFIO for `File > Open`).

Adds a LegacyTextDisplayViewer to display text output. This is necessary for displaying a Welcome to ImageJ2 message.

Added a plugin which, if any ImageJ2 options are unset (e.g. the first time launching the legacy UI) shows the welcome to IJ2 message, and also pops the display to toggle SCIFIO on or off.