### 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)

_change_dir(root_prefix.path_join(p_dir));
	file->set_text("");
	_push_history();
}
void FileDialog::_file_submitted(const String &p_file) {
	_action_pressed();
}
void FileDialog::_save_confirm_pressed() {
	String f = dir_access->get_current_dir().path_join(file->get_text());
	emit_signal(SNAME("file_selected"), f);
	hide();
}
void FileDialog::_post_popup() {