[Enhancement] PySimpleGUI-TK Ability to force window focus

When running multiple windows, serially where one is closed before another open, on Windows,
new windows do not receive focus after the first window. Only the initial window is given focus after creation.

Where is a workaround available. Use this when you create your window:

   window.TKroot.focus_force()         # Force this window to have focus

   window.Element('-FIRST-INPUT-FIELD-KEY').SetFocus()   # Force this Element to have focus

This will focus the window and move the cursor to the indicated Element.

Empieza reproduciendo el comportamiento de Windows con ventanas PySimpleGUI-TK creadas de forma serial, y después lee los puntos de entrada de focus existentes indicados en el issue: window.TKroot.focus_force() y Element('-FIRST-INPUT-FIELD-KEY').SetFocus(). Se considera terminado cuando las ventanas recién creadas reciben el foco de forma consistente sin requerir este workaround, mientras el elemento indicado todavía puede recibir el foco del cursor.