Editor windows difficult or impossible to access in single window mode

### Tested versions

- Reproduced in: 4.3, 4.2.2

### System information

macOS

### Issue description

in certain situations where a few subwindows are stacked, it is difficult or impossible to reach one of those windows if it gets hidden behind the others - if you accidentally click another one for example.

### Steps to reproduce

1. have a small screen or small editor window
2. enable single window mode
3. disable native file dialogs
4. open the export menu
5. open one of the file pickers
6. create new folder
- you should now have 3 embedded subwindows floating around, the create folder one being tiny
8. click one of the larger subwindows, bringing it to front and hiding the tiny window

if you are unlucky, your screen is too small for the larger windows to be moved and resized out of the way (because the subwindows are bound by the editor window size) to get the small create dialog - and it is impossible to interact with the larger ones while that dialog is open

i could think of multiple way that this situation could be avoided
1. allow closing the parent window while the subwindow is still open (file picker while create directory is open, export while both of them are open)
2. allow moving the subwindows out of the main editor window
3. closing the folder create popup when the file selector regains focus
- there may be other similar situations that are missed by this though
3. allow these blocking windows to shrink further, blocking less space. seems very flimsy to me though

### Minimal reproduction project (MRP)

editor only

Start by reproducing the stacked embedded subwindows in Godot 4.3 on macOS using the listed single-window, non-native-dialog steps. Inspect the editor's subwindow focus, sizing, and movement behavior around the export file picker and create-folder dialog. Done means a hidden blocking dialog can be reached or the proposed window interaction is made workable on small editor windows.

Written by the indexing model from the issue text.