Add methods to OhmNode and add case labels

[felixauringer]

• We added methods to check whether an OhmNode has no content. If that is the case, it is safe to ignore the node and its children when building the source map. This prevents all index out of bounds errors in the source map that we have found so far (all our comment-tests are green with this branch).
• We added a method firstChild to OhmNode because we need this operation very often.
• We added some case labels in order to write SemanticActions for these cases.

[coveralls]

Coverage increased (+0.03%) to 85.271% when pulling 774c995 on felixauringer:swt-submission into d7f2ac8 on hpi-swa:new-whitespace-handling.