Add API to access the "usesScrollButtons" property of tabs in a QMainWindow

Dock Widgets in a QMainWindow can be arranged using tabs.
If there are many widgets in a tab, one can set the "usesScrollButtons" property on the tab to allow scrolling rather than eliding.

This feature cannot be accessed in tabs that dock widgets inside a QMainWindow, although other properties can be controlled, e.g. documentMode, tabShape, etc.