Allow bevy_ui to handle multiple window

[fadhliazhari]

What problem does this solve or what need does it fill?

If we create multiple window in bevy, a bevy_ui entity will be rendered in both provided that they each have been assigned a camera.
Currently the focus system consider Interaction only for the primary window's cursor position.
This will not allow any bevy_ui Interaction handling for windows other than the primary.

What solution would you like?

Includes all windows that have bevy_ui to handle Interaction.