Add Horizonal Split Mode to the editor FileSystem dock

[AndreSacilotto]

Describe the project you are working on

A project with too many folders

Describe the problem or limitation you are having in your project

The "Split Mode" option on file system only split vertically and since most monitors have more horizontal space than vertical, would be more reasonable to be able to split horizontally.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add a tree toggle state to "split mode":

1. No Split
2. Split Vertically
3. Split Horizontally

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Just as it already is, with one additional state

If this enhancement will not be used often, can it be worked around with a few lines of script?

No - Editor Feature

Is there a reason why this should be core and not an add-on in the asset library?

Editor Feature

[Calinou]

While most monitors are widescreen nowdays, docks are generally not resized to be very wide. As a result, horizontal splitting in the FileSystem dock may not be very usable on most displays (unless you have an ultrawide display and are willing to compromise on space for the main view).

For instance, this is the default dock width when selecting the Default editor layout in the Editor menu:

Vertical mode would result in most of the names being trimmed unless you resize the dock to be much wider.

Also, since this would require turning a button into an OptionButton (to offer 3 different options), this would require one more click to switch between normal and split mode. This may or may not be an issue; I'm just pointing this out as it can be an usability regression for some.

[kitbdev]

It might be better to automatically detect if the FileSystemDock is wider than it is tall and automatically change between vertical and horizontal split mode.

[kitbdev]

implemented by godotengine/godot#80241