Gui: support toolbar drag and drop to status bar and menu bar

[realthunder]

Closes #12979

[PaddleStroke]

Thanks for bringing in these changes !

Few issues I have noticed :
1 - The sketcher 'edit mode' toolbar are present all the time in all workbenches :

The only way to remove them is to edit a sketch, which hides them. Then they stay gone until restart.

2 - There is no way to remove toolbar from the menubar. Once a toolbar is dropped in there it cannot be removed. We would need at least the same menu as in the status bar 'Unlock toolbar':

3 - The tabbar is oriented South instead of North when dropped in the menu bar :

4 - The tools icon size is relatively big compared to the menu and tabbar which does not look great :

Maybe there are some paddings or margins that could be removed. Or alternatively reducing the icons from 24 to 20px when in menubar or status bar? Here how it would look with 20 px icons :

5 - When dragging to the menubar, if the mouse is too close to the window border, things get weird :

And can get even more weird on some occasions :

[realthunder]

Add a few fix, please check again.

[PaddleStroke]

Thanks!
1, 2, 4 and 5 are fixed.

• 3 is not fixed yet. I think it's just the tab bar that defaults to south so this should be easily fixed.

One more small things :
6 - the preference 'Workbench Selector Position' needs to be removed.

• Currently if your position preference is set to right corner, the workbench toolbar does not appear at all, you need to set it back to toolbar and reboot. So if you remove the setting, don't forget to remove where the preference is used as well.

[realthunder]

More fix. Please check again.

[PaddleStroke]

Thanks all fixed now!

[Syres916]

@realthunder please see https://forum.freecad.org/viewtopic.php?p=758767#p758767 regarding new users regression.