gui: improve resource handling

I've only done a quick dump of the new assets. We should:

• detect desktop theme, and choose iconset accordingly.
• choose monocrome on mac?
• rename resources so they match leap_assets names.
• maybe separate main window / systray icons?
• some green somewhere?
• do something for the window icon.

(from redmine: created on 2013-01-29, closed on 2016-05-04, relates #2643 (closed))