When multiple windows are created, the tray functionality only works for the main window. When the main window is closed and only additional windows remain, the tray functionality will break.

The changes include:

1. Monitor all windows, and the tray functionality will default to the last focused window.
2. Allow showing/hiding each individual window.
3. Clicking the tray icon will show/hide the last focused window.
4. Add the ability to open a new window.
5. Fix for [issue TriliumNext/trilium#5551](Restore the existing window (if minimized in System Tray) when a new application instance is launched Trilium#5551).

I don't know where to find and convert the PNG for open new window; I downloaded it from www.iconfinder.com.