Add support for super-sources

Super-source shouldn't be added as source folders, for various reasons:

• it's likely that the IDE will use the super-source version for
• if you have a non-super-source and super-source versions of the same class, its likely the IDE will be confused, and flag them as duplicates
• in a typical setup, M2Eclipse compiles classes (using Eclipse JDT) in the same output folder as Maven (target/classes), so your super-source classes would be not only used as super-sources, but also as compiled classes in DevMode and Compiler: likely to fail!

At least all of the above are true in Eclipse.

It can be useful to temporary add the super-source folders as source folders to benefit from a larger set of features from the IDE, but it's a bad idea to do it by default (Eclipse has improved in the latest versions though when editing a Java file outside a source folder, so it might no longer be needed at all)

I believe super-source should just be seen as resources (which they are), but the kind of resources that only apply to compiling a GWT project (just like the *.gwt.xml and *.ui.xml files).

i had the same Problem with the super-source in eclipse with maven.
What Thomas said seems plausible, but i don't had any problems having the "emul"-folder on the buildpath yet.

Anyway, i have written a M2E Plugin for fix this problem:
https://gwt-m2e.googlecode.com/

It exclude the supersource from the buildpath and adds it as a new build-folder.
If anyone knows how to add a Preferences-Part into an eclipse plugin, we can make the adding part optional, so it only excludes to remove the eclipse-compiler errors.