Improved threshold nodes

[RunDevelopment]

Changes to Threshold:

• Use enum input for threshold type.
• Make max value conditional. It's only used for binary and binary inverted thresholds.

Changes to Threshold (Adpative):

• Use enum inputs.
• Reordered inputs to match Threshold node.
• Renamed "Mean Subtraction" to "Constant Subtraction" and changed its range to -100 to 100.
• Added docs.

This also includes a breaking change. Constant Subtraction in Threshold (Adpative) was previously an 8-bit value. This leaked the fact that we use 8-bit images under the hood, so I changed to 0-100 to be consistent with everything else in the threshold nodes.
I did not write a migration for this, because it's a pain, and because I am certain that nobody uses this node. It's complex, confusing, and undocumented, so I don't feel bad breaking chains that use it...