Port from QQC2.Dialog to Kirigami.Dialog
The Kirigami version is nicer and features a darkening overlay, standard styling for headers and footers, and a standard implementation of footer buttons. Adopting it modernizes the visual styling of the app and lets us remove a lot of boilerplate code.
There are more code cleanups and design changes that I think could be made here (for example, no longer overriding the dialogs' y positioning and just using the standard centered style) but I don't consider those in scope for this MR.
I tested all the features that I could find in these dialogs to make sure they didn't break.
NOTE: I was unable to test the changes to the Load Preset and Missing Sensors dialogs, as the parts of the app that can trigger these dialogs are currently broken due to not having been ported to Qt6 yet. Nevertheless, porting them was straightforward and followed the pattern in other dialogs that were tested, so I have reasonable confidence that they work
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