deleting large selections in C++ files takes ages

To reproduce: open qwidget.cpp, select all, delete. The operation will complete eventually, but take a very long time.

This is related to the increased QTextCursor usage by the C++ editor. Profiling shows that most of the time is spent in QList<QTextCursorPrivate*>::contains, called by QTextDocumentPrivate::adjustDocumentChangesAndCursors. Maybe make changedCursors a QSet if the order does not matter?

This problem does not exist in 2.0.0.