In a filesystem dock in split mode, the file view mode icon shows the next mode instead of the current mode

### Tested versions

Reproducible in 4.2.1.stable

### System information

Godot v4.2.1.stable.mono - Windows 10.0.19045 - Vulkan (Forward+) - dedicated Radeon RX 580 Series (Advanced Micro Devices, Inc.; 31.0.21905.1001) - AMD Ryzen 7 2700X Eight-Core Processor (16 Threads)

### Issue description

More of a UX inconsistency than a bug.

In the filesystem dock, the icon/button that has focus in the image below shows the state the panel will be in if you click the button.

In itself this isn't a problem, but most other buttons/icons in the editor show the state the element is currently in, not the state it will be in if you click it.

My expectation here is that everything in the editor should follow the same UX conventions. In this case, buttons/icons should show the current state (and not the next one).

Not a huge deal but I was a bit confused by the current behavior at first. IMO changing it to fit the rest will help avoid confusion for other/new users.

### Steps to reproduce

To make the icon/button in question appear in an empty project, click the white square in the filesystem dock to switch it to split mode.

### Minimal reproduction project (MRP)

N/A

Reproduce the behavior in an empty project by switching the Godot editor's filesystem dock to split mode, then inspect the filesystem dock implementation and its mode-toggle handling. The change is done when the icon shows the current view mode consistently with other editor controls, and the existing reproduction no longer shows the next mode.

Written by the indexing model from the issue text.