Add IJ.open hook

It is not sufficient to intercept the openImage method - we want to give
IJ2 a chance to open all resources (e.g. scripts). So we need to
intercept at the IJ.open level in addition to openImage.

Note that IJ.open is void and assumes whatever method it delegates to
handles displaying the opened resource. Thus the hook interceptOpenImage
method returns a boolean to indicate if the provided path was handled or
not.
Update LegacyOpener use

Consolidated to a single method signature with a display flag to allow
differentiating the desired functionality.
Remove outdated intercept ref

There is no interceptOpenImage any more.. just interceptOpen.
Javadoc fix: LegacyHooks

Fixed variable name.
LegacyExtensions: update install method name

Changed installOpenImage to installOpen, as the hook is now broader in
scope that just images.
Merge remote-tracking branch 'origin/broaden-opener-scope'

This branch kicks wide open the door to use SCIFIO by default when
calling File>Open in Fiji!

Thanks, Mark Hiner!

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>