Also check for icons in the assets.

Netzke now uses `assets/icons` as the default path
to find the icons, so we need to look for the icons
in the asset-folders. Since the icons are supplied
by a vendor, I place those generally in `vendor/assets`
so for completeness check all asset paths.

This will still fail if the icons are bundled in a
asset gem.

I can still clean up the code a bit, and add tests, but I needed this code myself when I moved my 
icons from `public/images/icons` to `vendors/assets/images/icons`. All the action-columns worked, but the
buttons in toolbars suddenly lost their icons.

This change fixes that.