(Backport) Issue #491: Script-fu support for all BMP save features

Hopefully finishing what I started 4 years ago (#491 (comment 209247)) and resolving issue #491 (closed).

This adds a second command, file-bmp-save2, which allows non-interactive control of whether the color space is written or not. It also adds the other two BMP save options, whether to use Run-Length Encoding and what RGB Format to save the pixels in. The prior patch has been revised to follow the parameter order of 2.99's file-bmp-save for eventual easier updating to 3.0.