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.