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).

[JES: reimplemented after the recent initializer changes]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
DefaultLegacyOpener: respect useSCIFIO option

The useSCIFIO option will now turn the DefaultLegacyOpener on or off.
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.
ImageJ2Options: add getter

Added a public isUseSCIFIO accessor.
DefaultLegacyOpener: simplify isUseSCIFIO logic

Updated to use the API recommended by Curtis Rueden.
Remove unnecessary persist=false

Service @Parameters do not need to be labeled persist = false in
OptionsPlugins.
ImageJ2Options: update plugin type

Annotated as a LegacyCompatibleCommand, this plugin wasn't being
discovered as an OptionsPlugin. By annotating on the concrete type, it's
now discovered as both.
TEMPORARY bump to latest component versions

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Show IJ2Options on WelcomeEvent

Added an event listener to display the ImageJ2Options in response to a
WelcomeEvent.
Create LegacyOpenDataset

Adds a new OpenDataset implementation to override the DefaultOpenDataset
implementation.

This just adds a "Read more..." button to the import window which will
open a link to a page explaining the use of SCIFIO in IJ2.
Run source > Clean up on display package

Applied IJ2 style template via Source > Cleanup in Eclipse.