Qnx: implement "Newest API level" option

The "Newest Api level" virtual option matches the most up-to-date version of
the installed APIs. If a more recent version is then afterwards installed, it
will point to it. Likewise, if the most recent version is uninstalled, it will
default to the next most up-to-date version.

Change-Id: Ic0609df92ff9108f897559e80a108e22c6bc11a0
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>