[#3505] use QT APIs screenAdded and screenRemoved to reiniitalize Fla…

[leehambley]

…meshot when the display configuration changes

[Saibamen]

#3505 (comment):

@leehambley Tried it, it does detect the external monitor (even after unplugging and plugging again) 👍

However, it introduces a couple of regressions:

• The screenshot capture on the external monitor does not show any open windows. You can only screenshot an empty screen.
• It adds an additional window on the main monitor - to now dismiss the screenshot mode, you have to press esc -> click on the main monitor to focus -> press esc again.

Tested on MacBook Air M2 with 1 external monitor.