Fixes #1001326

This commit fixes ticket #1001326. Solved the problem by
overriding 'parseTree(source: SourceFile)' in
ScalaPresentationCompiler with an implementation that just
parses the source every time without trying to memorize
anything. Added tests that checked that

  1. You get a new parse tree every time you ask for one
  2. A parse tree should never contain any symbols or types [1]
  3. If you ask for a parse tree and then open the file it shouldn't
     change the parse tree you originally asked for, i.e. property
     2 still holds.

[1] There is an exception to this though. Some of the nodes that
    the compiler generates will actually contain symbols. I've
    chosen to just ignore these special cases for now.