Support for missing icons

[metbril]

Some icons in the classic iconset are still missing in the MDI repo. During generation there are 2 options: use a stub icon that is used for any missing icon in the iconset or don't create an icon at all.

The current script supports this by defining icons (or not) in the yaml file. But perhaps we could add some configuration and a command line option to choose.

For example:

--missing help

for using the MDI help icon for any missing icon