Fix issue #1003

[philippejer]

See issue #1003

Problem

For the VB to CS project file conversion, <DefineDebug> and <DefineTrace> should be converted to <DefineConstants>.

Solution

• I've used XDocument to parse and edit the XML (this is already used elsewhere in the codebase, e.g. to parse the resource files).
• I've also done a similar job in the neighborhood for consistency and to simplify the code (especially in the case of the <DocumentationFile> conversion).
• The main change in external behavior (aside from fixing the issue) is that the XDocument step can cause minor changes in formatting (this can be seen in the updated test files), but I don't think this is an issue (they are new files generated by the converter, not existing files).
• I've done a small change in MultiFileTestFixture to ouput the expected file path in case of failure.
• The code is already covered quite well by the tests, I've updated the relevant test cases.

[GrahamTheCoder]

Great job, thanks for this, it's much nicer parsing the xml than purely using regexes everywhere