WindowServer: Save config file when mouse cursor theme is changed

When user code requests the current cursor theme name with
`GUI::ConnectionToWindowServer::the().get_cursor_theme()`, that reads
the name from the config file. If we don't write that out when it's
changed, then users get given an outdated cursor theme instead of the
current one.

In particular, changing the theme in MouseSettings, saving and closing
it, then reopening MouseSettings, would show the wrong theme selected.
So, that's fixed now. :^)

Userland/Services/WindowServer/WindowManager.cpp

@@ -2260,6 +2260,9 @@ void WindowManager::apply_cursor_theme(String const& theme_name)
 
     Compositor::the().invalidate_cursor();
     m_config->write_entry("Mouse", "CursorTheme", theme_name);
+    if (auto result = m_config->sync(); result.is_error()) {
+        dbgln("Failed to save config file: {}", result.error());
+    }
 }
 
 }