Improve floating docks with multiple dialogs (reopening dialog creates blank window)

Steps to reproduce:

• open Inkscape
• undock a dialog (A)
• undock another dialog (B) and dock it to (A):
• Title bar changes to e.g. Dock #3
• Close the dock
• Reopen any of the dialogs (A or B) that had been docked within this window

What happened?

Dock reopens with its former size, but is empty and can't be closed without restarting Inkscape. Impossible to bring up the dialog again.

docked_dialogs

After restarting Inkscape, the dialog configuration is forgotten; both dialogs (A and B) open in their own window.

What should have happened?

• After opening one of the dialogs (A), the dock should open again - either with only the dialog (A) or both of them (A and B)
• After restarting Inkscape, the configuration should still be there.

It is quite handy to keep a more complex group of docks on in a separate region, e.g. a second screen.

Inkscape Version and Operating System:

• Inkscape Version: 1.1-dev (2a5d4770, 2020-01-18)
• Operating System: Windows 10
• Operating System version: 10.0.18362