While Oxygen draws no frames around toolbars, yet it had the style hint
for the frame width set to 2, while having the actual toolbar margin
set to 0.

The possible cause is that PM_ToolBarItemMargin was only introduced for
Qt4, so seemingly Qt3 times' workaround to set the margin via the frame
width missed to be adapted.

Qt's own codee uses PM_ToolBarItemMargin & PM_ToolBarFrameWidth to
calculate the QToolbar layout's contentmargin, so switching the values
results in no actual change there.
The only other use is PM_ToolBarFrameWidth for initializing the
QStyleOptionToolBar::lineWidth, which though is unused by Oxygen drawing
code, so the change also has no effect.

But 3rd-party code querying the pixel metric for these two values after
this patch gets proper values and can properly align custom UI painting with the
styled toolbar.