Save a denoised frame when clicking "Save current frame"

Currently, the denoiser only denoises the image after it has been rendered to the full configured SPP.

Originally raised by @NJDaeger:

> I was wondering if that button could be changed to either save the denoised image by default or to give the option to save the denoised image vs the non denoised image
either that or another button be added into the denoiser tab to save the denoised version of the current frame
which would probably make more sense

Start by locating the "Save current frame" control and the denoiser tab in the Java plugin. Trace how the current frame is saved and how denoising is applied after the configured SPP is reached. Done means the control saves the denoised current frame, or clearly exposes the requested denoised-versus-original choice.

Written by the indexing model from the issue text.