Threshold application data type might be wrong

[ctrueden]

When you select the Apply button in the Threshold plugin the pixels are changed to 0 or 255. This matches IJ1. But IJ2 supports true 1-bit masks. Perhaps we should be creating 1-bit data here. On a related note we could just have Apply call the convert to mask plugin.

Migrated-From: http://trac.imagej.net/ticket/1685