Show a warning when using relative icon paths

[johannhof]

In https://github.com/mozilla-jetpack/jpm/blob/master/lib/rdf.js#L90 we use a hack to replace relative icon paths with absolute resource:// URLs, since relative icon paths are not supported in Firefox (yet?).

This should not be the default way to go because:

• resource:// is not guaranteed to be around forever
• (more importantly) the icon will not be displayed if the add-on is disabled

Hence we should log a short warning that it is recommended to use a file called icon.png / icon64.png in the root directory instead and link to https://developer.mozilla.org/en-US/Add-ons/Install_Manifests#iconURL

[yan12125]

Maybe also provide another option to use chrome registrations? Personally I don't think forcing everyone to put icon files on the top level is a good idea.

[Croydon]

I feel like the best solution would be to fix this in Firefox.

[johannhof]

Firefox fix is tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1141839