Add optional second image input to "VIEW IMAGE" node and a slider for "before/after comparison

[Wetzer2]

Motivation
It makes so much sense to have these "before/after comparison" functionality in a program like chaiNNer where most of the time existing pictures were improved with varying results. It would make tweaking parameters for e.g. upscaler more easily.
BTW: Original idea from SharekhaN: https://discord.com/channels/930865462852591648/930865780663398451/1244330228218724453

Description
Add optional second image input to "VIEW IMAGE" node and a vertical slider (only present if second input is used, optional also a horizontal and 45 degree tilted version) for switching the output view between input one and input two of the node.
Optional functionality: an output of the before/after image.

Alternatives
I don't see any for that scenario.

[joeyballentine]

At some point we are going to make image comparisons its own node