Keep icon sizes as specified by the user but clamp them to the button size.
Fixes #857 (closed)
Tested it in all panel and plugin modes under X11.
Assumes a fixed padding. Ideally it would be based on Gtk style. Having said that, the existing version uses hardcoded icon sizes too, often larger than button sizes, so it is still an improvement.
No longer uses nrows_ratio. Second commit optionally removes nrows_ratio definitions.