Give the ResizablePane the option to add a toolbar

[FelixMethe]

Use case

This would make it possible to add different toolbars to the ResizablePane and ContentArea like the mail app does for example.

This is not possible now because the ResizablePane adds a MacosScrollbar over the content of the pane.

Proposal

I propose to add an optional toolbar argument to the ResizablePane that adds the toolbar above the content of the pane.

Please let me know if this is a desired feature.