Add new "New File Wizard"

For more information about this wizard see the proposal that can be
found in the associated ticket.

Fixes #1002103
Add combo box that can show images

The implementation is copied from the nebula project. A link to the
source and some examples can be found in the copyright comment of the
code.
Validating logic now checks Scala and Java keywords

Previously it was only checked if identifiers can not correspond with
Java identifiers which is not enough for Scala identifiers.

Furthermore package names are not allowed to contain Java identifiers.
The Eclipse package hierarchy view can't display them correctly and even
the Java compiler of JDT seems to have problems with them.