Outline feature

The current outline representation is quite useless and contains a lot of unnamed elements.

This implementation shows some meaningful labels and icons, where the icons are taken from public domain icons from the [Tango Project](http://tango.freedesktop.org) project. Feel free to replace them to anything you like better.

I tried to adapt the checkstyle rules, however I would argue about that method name constraint in particular, as I would suggest allowing the '_' character as an allowed prefix for method names as they are used for XText PolymorphicDispatch Reflection Util.

I would still recommend to switch away from the outline view when editing a file, as currently (which I left unchanged) the tree is always completely reorganized whenever something in the file changes.
For viewing or searching in files however this should be useful.

This pull request also might affect issue #2 ...