Workaround MSVC2008 build failure after switching to std::upper_bound

Workaround MSVC2008 build failure after switching to std::upper_bound

QPostEventList is kept sorted by priority; std::upper_bound
is used to insert a QPostEvent in the right place in the list.

Turns out that MSVC2008 is a bit too picky and tries to see if the
list is actually ordered. This causes a build failure as there is
no operator< defined between two QPostEvents (in fact,
an integer -- the priority -- is passed to std::upper_bound).

Work around this issue by defining operator< between two QPostEvents.

Change-Id: Ie3562dd0cc7253e25fc988b25d566d9d9e9fe62b
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>