Mac: Update unified toolbar drawing code.

Fill the toolbar background with transparent
pixels to allow the view background to shine through.
Draw a separator line using HITheme.

Task-number: QTBUG-34411
Change-Id: I57732564a67a5a9d1eb7fd9038382b2b65be50a4
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Mac: remove unified toolbar workarounds.

QMainWindow::unifiedTitleAndToolBarOnMac() will
again return true, but the new implementation is
much more similar to the cross platform one so these
workarounds are not needed.

Task-number: QTBUG-34411
Change-Id: Ic95f491af94354776385dc984f1a00c64429a3f0
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>