Multimonitor fix (#4498)

* Rework capture system to only enter fullscreen widget on one monitor

* Sort monitor previews by screen position instead of Qt index (#4522)

The monitor selection dialog displayed monitors in Qt's default
screen order (by connector name, e.g. DP-1 before DP-2), which
can differ from the physical arrangement configured in the display
settings. This sorts the preview widgets by their X geometry
position so they match the user's actual left-to-right layout.

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Co-authored-by: Brian J. Cohen <brianjcohen@gmail.com>