QTime::elapsed() is deprecated in recent Qt5, and removed in Qt6. Use
QElapsedTimer instead, which is the suggested replacement.

vsprintf() is deprecated in recent Qt5, and removed in Qt6. Its
replacement vasprintf() is available since Qt 5.5.

fromTime_t has been deprecated and removed from Qt6. Use the replacement
fromSecsSinceEpoch() on Qt 5.8+.

QRegExp is deprecated in recent Qt, and has been removed in Qt6. Use the
replacement QRegularExpression.

fromList() is deprecated and has been removed in Qt6. Since Qt 5.14
range constructors are available, use those instead.

FileAppender will no longer call QFile::flush on every message write
due to it dramatically slowing apps when writing a lot of log records
to slow file system. The downside is some log data may be lost if
application crashes before QFile manages to write all of the buffered
data to file system.

It is still possible to revert to an old behavior calling
setFlushOnWrite(true) on FileAppender.

CMake started showing deprecation warning about CMake 2.8 files
support.

Qt6 moves QTextCodec to the Qt5Compat module. Since it's unused anyway
remove it so we don't get the dependency on Qt5Compat.

Getting the values from a QMultiMap() returns a list, but not the same
on consecutive calls. Thus calling it twice to get the iterators will
yield different lists, causing the iterator to fail.

Fixes segfault when destructing the logger.