Don't assign an initial widget for dock items (fixes initial sizing)

Adding all the dock widgets to the right dock area at initialization sets an initial size hint on the ShellWindow that forces the window to a large default size (~2291 px height) which overrides the user window height that would otherwise be automatically restored.

Not adding the widgets to the dock area at this stage does not prevent them from being correctly restored later if the user's config asks for it. It also doesn't break the initial appearance of the app, as we automatically load a default config when one is not present.

BUG: 462703

FIXED-IN: 0.26.17