Creating source: file location (.) and extension (cpp) not configurable

[hattesen]

When choosing "Create implementation" from the context menu, where no source file exists, a new file is created in the context directory, with a .cpp file extension.
Changing the order of the "CppHelper.SourcePattern" setting makes no difference.

Proposal

1. Obtain the default default source file location and extension from the first "CppHelper.SourcePattern" entry , e.g. "/src/{FILE}.cxx" or "{FILE}.cpp"
2. Create two new settings for "Default source file path" and "Default source file extension".