### 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)

switch (p_what) {
		case NOTIFICATION_VISIBILITY_CHANGED: {
			if (!is_visible()) {
				set_process_shortcut_input(false);
			}
			invalidate(); 
		} break;
		case NOTIFICATION_THEME_CHANGED: {
			dir_up->set_icon(theme_cache.parent_folder);
			if (vbox->is_layout_rtl()) {
				dir_prev->set_icon(theme_cache.forward_folder);
				dir_next->set_icon(theme_cache.back_folder);
			} else {
				dir_prev->set_icon(theme_cache.back_folder);
				dir_next->set_icon(theme_cache.forward_folder);
			}