Save gutter images and folding dots with transparent background

This makes them conform to the informal template image format which is usable as-is as a CoreGraphics image mask.
Skip explicitly creating a CoreGraphics image mask
Use CGImageRelease instead of CFRelease
Add warning and error gutter view images
Use string instead of double for mate’s version number

With a double we can’t use version numbers like 2.8.1 or 2.10.
If updating mate requires admin privileges then ask user first

This way they should know what the password dialog is about.
Construct undefined text::range_t/pos_t if created from NULL_STR
Don’t change ninja goal for the mate target
Add API to remove (all) marks from (all) documents
Add support to mate protocol for setting and clearing marks
Add --set-mark/--clear-mark options to mate