Change max button width on per-app-ID basis

multirow_equal_width option made me wonder - would it be possible to add the ability to override maximum taskbar item width on per-application basis? I find that for some applications I cannot disable labels because the title change indicates notifications (like having "[1]" in front) but there's no reason to care for anything past first 20 pixels of text, thus the taskbar button width could be cut in half (or more) to free up space for other things.

This would also allow to make select application buttons wider to accommodate long labels, which might be an easier way around than implementing label measurement mentioned in #437.