icon: Update app icon

Notes:

• I've removed HighContrast icon, if it is still needed we can symlink symbolic icon.
• Win icon generated from svg file, looks nice on my end, but I cant test it on Windows. File data/icons/Makefile adjusted.
• Probably, data/icons/README requires more changes.
• File meld/build_helpers.py refers to png file - probably, it can be removed (I didn't change it).

            for size in glob.glob(os.path.join(theme, "*")):
                for category in glob.glob(os.path.join(size, "*")):
                    icons = (glob.glob(os.path.join(category, "*.png")) +
                             glob.glob(os.path.join(category, "*.svg")))
                    icons = [

• Maybe I missed something else.