Uses of the \compat command should be flagged with a qdoc warning

There remain many uses of the \compat command in the C++ sources. Typically, a member function is marked with \compat in its qdoc comment, when that function is meant to be used for maintaining compatibility with Qt3. i.e., the function is part of Qt3 support. But compatibility with Qt3 is no longer supported in Qt5, so these functions should either be marked obsolete and eventually removed, or the \compat command should simply be removed. For now, qdoc will print a warning for each use of compat but continue generating the same html output for the functions marked with \compat. Developers should then decide whether to remove the function or the \compat command.