Fix the synchronous-mode wallet open logic

BUG: 254198

The wallet synchronous open requests now use qdbus delayed replies.
The execution path now returns to the main event loop instead of
a nested event loop. The wallet opening UI logic is correctly handled
no longer leading to a frozen kwalletd.

Beware that this commit should be used along with the corresponding
fix of the kdelibs/kdeui module. Failing to updating kdelibs lead to
an aparently similar freeze condition, as kdeui also used an internal
event loop, faking synchronous operation when making async kwalletd calls.