Generalize incremental parsing maintenance

[mcserep]

All plugin parsers should be able to perform maintenance operations related to incremental parsing.

• Extend the AbstractParser type with a virtual void maintain() {} method to support operations related to incremental parsing or other maintenance.
• Refactor the incrementalParse() method of the CppParser type into the parser's maintain() method.
• In parser.cpp, call the maintain() method for all parsers before calling the parse() method. The plugin parsers should be traversed in a reverse topological order, hence inverting the dependency relations defined between them for parsing.

Blocked by #8.

[mcserep]

As decided on the Ericsson CodeCompass meeting today, the maintain() method should be named preparse(). The preparse() method can read the file statuses from the ParserContext and decide to detect further status changes, e.g. through the C++ header inclusions (see #8). The preparse() is also responsible for the database maintenance as it was originally designed.

[intjftw]

Solved in 8f85585