Qt: Migrate to PySide 6 (Qt 6.0+)

[kristapsk]

PySide2 (Qt5) is causing problems, especially with Python 3.10, see #1031, #1075, #1343.

Potential solution seems to be migrating to PySide6.

Tried to work on that direction some time ago, but stopped with this error - https://github.com/JoinMarket-Org/joinmarket-clientserver/pull/1046#issuecomment-1013018537. Not sure how big code changes are needed.

Qt6 seems to be supported by the most recent OSes, like Ubuntu 20.04, macOS 10.14+, Windows 10 (1809 or later).

[kristapsk]

This might be useful - Porting Applications from PySide2 to PySide6.