Replace const char * based connect statements by modern API and remove
'_q_' slot prefixes.
This requires explicit disconnection in the destructor -> add.
Replace bool traps for layouting and removing buttons by enum classes.
Replace if/elseif event handler by switch.
Replace iterator typedef with auto.
Encapsulate logic to ensure defaulting to first accept button in a
member function for better readability.
Remove dead code.
Move private header declaration from cpp to a new _p.h.
Task-number: QTBUG-114377
Pick-to: 6.6 6.5
Change-Id: I8a2bc355e3816c3c826c10cd96194c89bf0ae510
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
df735d794f