Fix lost input events with popups and multiple windows on linuxbsd

[Sauermann]

fix linuxbsd variant of #66016

In the following setting

• Root Window (don't embed windows)
  • Popup
  • Secondary Window

Mouse-Button-Down events in the Secondary Window were lost, because they were used for trying to close the Popup.

This PR makes DisplayServerX11::mouse_process_popups aware of multiple windows and tries to close them only if no other window was targeted.

Please review with care, because I am touching areas, I am not familiar with.