Add option to force status bar separator to be drawn

Originally when introducing the frameless feature in Breeze, we draw a
separator for all the QStatusBar. It turned out that some applications
where using QStatusBar outside of QMainWindow::setStatusBar and this
broke stuff for them, so we changed to only draw a separator for status
bar of main windows which covers almost all the cases.

Dolphin is another exception where a QStatusBar is draw for each split
windows and as such is not part of the QMainWindow. In Dolphin we do
want to have a separator between the main content and the status bar, so
introduce another style hint for this case.