Make settings decorator correctly set notify signal.

Previously made wrong assumptions about the use of the notify
signal in Qt properties. It is actually meant to expose the
signal we commit to emitting when our setter is called. Not
a way to tell Qt to emit this signal for us when the value
changes. Accordingly PYSIDE-261 was closed as invalid.
Fix insert item unit test broken by new AND Element.
Fix linter error due to __all__ for symbols package not being a tuple.

A tuple with one element needs a comma at the end.
Fix sporadic modal dialog test failures by adding delays.

Unfortunately it seems like there can be differing delays
when opening and closing modal dialogs. On my local box this
might be caused by animations and other slowdowns which made
the issue reprodible locally for me by running the test-suite
in a loop. These additional delays hopefully also fix the
issue on the CI systems for good.