This fixes the toolbox layout for RTL by properly mirroring everything.
But I don't know if this is what RTL users really expects. In addition, it can be switched to LTR or RTL explicitly via the context menu:
In auto mode, when docked on the left or right side it automatically uses the LTR or RTL layout respectively, otherwise it follows the UI layout direction set by the language.
The vertical docker title bar being on the left is an oddball (probably a bug) in QDockWidget... (Reported upstream: https://bugreports.qt.io/browse/QTBUG-100323)
We will need some opinions from RTL users before merging this. (Done)