qt: Use Dialogs foregroundwindow code in confirm

* qt/pinentryconfirm.cpp (PinentryConfirm): Minimize and raise on
start.
(showEvent): Do not call raiseWindow.

Pinentryconfirm did only partially use the improved foreground window
code from a421ae8c4d2a0ce182e099e634e49ccdad454627.
The raiseWindow in the showEvent was too agressive, which could
result in attaching to a deactivated Window. Which Qt uses
with QWindowsWindowFunctions::AlwaysActivateWindow.

This is not neccessary anymore with the Minimize / Maximize trick.

• GnuPG-Bug-Id: T6135