### 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)

tree->deselect_all();
	
	if (!tree->is_anything_selected()) {
		get_ok_button()->set_disabled(_is_open_should_be_disabled());
		switch (mode) {
			case FILE_MODE_OPEN_FILE:
			case FILE_MODE_OPEN_FILES:
				set_ok_button_text(RTR("Open"));
				break;
			case FILE_MODE_OPEN_DIR:
				set_ok_button_text(RTR("Select Current Folder"));
				break;
			case FILE_MODE_OPEN_ANY:
			case FILE_MODE_SAVE_FILE:
				
				break;
		}
	}
}
void FileDialog::_tree_multi_selected(Object *p_object, int p_cell, bool p_selected) {
	_tree_selected();
}
void FileDialog::_tree_selected() {