Threshold plugin is not a singleton

[ctrueden]

In IJ1 there is only one threshold plugin open and changing from window to window changes the target of the threshold plugin. Changes aren't instantaneous but changes to dialog fields apply to the current image window.

In IJ2 the threshold plugin is mapped to one ImageDisplay. And you can run the dialog multiple times (1 per display). This might need to be made more the IJ1.

Migrated-From: http://trac.imagej.net/ticket/1683