QMainWindow: add layout option to prevent empty spaces between tool bars

It is possible to move tool bars around manually, making empty spaces between tool bars which may lead to usability issues, especially when saving and restoring tool bar states. It would therefore be nice if an option was added to layout tool bars in such a way that empty spaces could be prevented, and to prevent the user from making such empty spaces.

This would also make the behavior more similar to that of tool bars in Qt 3, which did not allow empty spaces between tool bars.