notify-send: Support commas in icon filenames

Do not cut off an icon filename (passed to -i) at the first comma character. The original motivation probably was to support a list of icon names in -i separated by comma. However, this has never been implemented and the help text for -i also only mentions "an icon".

Hence, no functionality is lost by this change. In contrast, -i now supports commas in icon filepaths.

This solves issue #16 (closed).