Better small screen support

[caimantilla]

When using a smaller screen, especially one which isn't widescreen, the Godot editor isn't entirely usable. This is especially relevant for people developing their apps on older monitors, tablets, or side-by-side with other apps on the same screen. With UI plugins enabled, there are issues even at 1280x1024, and 1024x768 is unusable at 100% scale no matter what.
This mainly stems from the use of box containers in the UI, which force a minimum size. Lowering the editor scale to 75% helps, but makes things very ugly, causes difficulties with pixel-perfect 2D editing, and still doesn't always offer enough room. This is especially true when using plugins which add new menus, making these containers even wider than they already were.
By using flow containers, however, this can be solved. It shouldn't make any difference if the screen is wide enough, but frees up space when the window isn't wide enough, enabling development on lower-resolution displays.
Current form is a little iffy, though. When changing the nodes used for the spatial viewport menu, I ended up with this monstrosity:

Well, I tried using a VBoxContainer instead, but then the different menus would be forced to stack, even if there's ample space:

I think this is a fair tradeoff for supporting more users, but I understand this might be divisive. I'd like input on this change, or a better solution if that's doable.
A comparison of the master branch vs edited on a 1024x768 screen at 100% scale:

• Production edit: This closes Use HFlowContainers for toolbars to allow for smaller resizing of the MainScreen element godot-proposals#7156.

[caimantilla]

(just saying , the changes need to be done to the 2d editor as well, but without a consensus on how to approach this problem there's not much point doing that yet)

[kitbdev]

This closes godotengine/godot-proposals#7156

[caimantilla]

did 2d editor.

i'll see about the script editor later. maybe the help buttons could be hidden inside a MenuButton if there's not enough space? a smidge more complicated but sacrificing vertical space when coding isn't ideal. i'd say the script editor isn't a concern because it can be popped out, but for development on a tablet (or even phone) i'd imagine that's not a viable option.