Godot: Editor UI is resized when window is resized

[TheFlamyy]

Godot version: eeee471

OS/device including version: Ubuntu

Issue description:
While the script editor is open, resizing the window messes around with the layout. Picture one shows the default maximized window, picture two the maximized window after making it smaller.

Steps to reproduce:
Create some "broader" docks. (Save the theme for faster resetting)
Unmaximize the window.

[qarmin]

Godot should have option to lock dock size

[capnm]

Godot should have option to lock dock size

I think the dock just should behave the same way in the editor case as it currently does with the open view.

[akien-mga]

CC @groud (I guess the bug might have existed before, but #23114 likely made it more prominent).

[avencherus]

On windows I've lately been noticing that the width of the file manager dock keeps changing size when I minimize or re-open the editor. Wondering if it might be related to this.

[groud]

Note to myself: we agreed with @reduz that splitcontainer requires an "internal" offset value. So that the real offset property would reflect the preferred (via dragging or set_offset) offset.