Copy / Save as PNG - width / height

[lpatiny]

When copying or saving as PNG we can now select the DPI:

• Select DPI when copying / saving as PNG ? #3228

But this makes only sense if we can as well specify width and height of the final image.

• Should we define somewhere if we use inch, mm or cm (general prefs of NMRium ?) or have a combined input with width / height + units (mm, cm, inch)
• We should have 2 fields for width and height
• We could have a drop down menu that fills width / height depending some predefined sizes like A4, A3, ... like for pdf exportation.

[targos]

As a reference, here's the interface to change the image size in Affinity Photo: