Add Qt 6 support

This commit series brings initial Qt 6 support to QCA while preserving compatibility with Qt 5 (but raising the required version to Qt 5.14 like most other KF5 components as well). Still a few occurrences of QRegExp and QTextCodec need to be replaced in order to drop the dependency on the Core5Compat module later. Besides this QCA builds fine with Qt 6.1 and passes all tests.

A few (mostly whitespace-related) minor changes were introduced to meet the output of the latest clang-format-11 version in debian/unstable.