Add new variables / image attributes for megapixels and file size

Feature request
Just a simple request to add variables and image attributes so we have more options for viewing and filtering, specifically related to image size.
Certain websites, print shops and other photographic services have a maximum/minimum megapixel count and/or file size, so the ability to view and filter by these criteria would be welcome.

We can currently view the image's width and height, so the megapixel count can be calculated, but it would be good to view this information without the need for a calculation, and there is no current option to filter via this information.

Proposed solution
Add all or some of these:

Variable: $(MEGAPIXELS.RAW)
Variable: $(MEGAPIXELS.EXPORT)
Variable: $(FILE.SIZE)
Image attribute: Megapixels (of image in its current state, i.e. after cropping if crop applied)
Image attribute: File size (original file on disk)

Start by tracing the existing width and height variables and image attributes mentioned in the issue, then inspect the entry points for RAW, EXPORT, cropped-image, and on-disk file metadata. Compare the available representations before deciding which requested variables and attributes can be supported; done means the selected megapixel and file-size values are viewable and filterable as specified.