Fix initial dock tabs

[KoBeWi]

Fixes #113155

But this is weird. The editor did not load the default layout previously, I don't know what was causing the first tabs to be active.

[Mickeon]

We could merge this as an urgent fix, but it would be much better to know what's going on because the sheer bewilderment on why this is even happening is not... great

[kitbdev]

• I think this was actually caused by Remove extra NOTIFICATION_VISIBILITY_CHANGED notifications in docks #113070.

Moving the p_dock->show() in open_dock to after it is already in the TabContainer caused TabContainer::_on_tab_visibility_changed to be called which automatically sets the changing tab to be the one being shown. This didn't happen before since it was visible before adding to the TabContainer, so the current tab was left at 0.
So open_dock's p_set_current is currently not being respected.

I think p_dock->show() can just be removed, since the TabContainer handles the visibility.

[KoBeWi]

Ok I made a better fix. The notification order still seems consistent.
CC @lodetrick

[kitbdev]

Looks good.

[Mickeon]

Excellent work @kitbdev

[lodetrick]

Looks good! It shouldn't mess with any of the notification issues because the closed dock parent is now hidden.

[akien-mga]

Thanks!