### Godot version

v4.1.stable.official [970459615]

### System information

Windows 10

### Issue description

When changing the `file_mode` of `FileDialog` from "Open File" to "Save File" via code the "Save" button keeps disabled until a file or folder is selected if the "Open" button was disabled before.

Basically the button activity has to be updated when `file_mode` is changed. I have made the change already but the last time I opened an PR without an issue some people requested I open one. So here you are!

### Steps to reproduce

- Open the MRP and start it
- click anywere to open the dialog
- after 10 seconds the dialog changes file_mode
- if the button is disabled before the change it is as well after it

### Minimal reproduction project

[file_dialogue_test.zip](https://github.com/godotengine/godot/files/11991311/file_dialogue_test.zip)

#include "file_dialog.h"
#include "core/os/keyboard.h"
#include "core/string/print_string.h"
#include "scene/gui/label.h"
FileDialog::GetIconFunc FileDialog::get_icon_func = nullptr;
FileDialog::RegisterFunc FileDialog::register_func = nullptr;
FileDialog::RegisterFunc FileDialog::unregister_func = nullptr;
void FileDialog::popup_file_dialog() {
	popup_centered_clamped(Size2i(700, 500), 0.8f);
	_focus_file_text();
}
void FileDialog::_focus_file_text() {