Logging: support %{backtrace} in QT_MESSAGE_PATTERN

On supported platform, allow to show a backtrace by using
%{backtrace} or %{backtrace depth=N separator="|"}

[ChangeLog][QtCore][Logging] QT_MESSAGE_PATTERN can include a
backtrace using %{backtrace}

Change-Id: Ib00418c070d3bd6fe846dc04bf69fa91ba64f9cd
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
tst_QFont: remove unused static function

This function has been inlined in commit 4f50be8d,
but wasn't removed.

Change-Id: I1a4eb7399ab2dd87a9fb15c4c886004d777487a8
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>