Fix glitches in certificate list

When opening the sign/encrypt or decrypt/verify dialog, the certificate list in the main window stops scrolling correctly;
the scrollbar continues moving, but the table's content does not move accordingly. (This seems to be specific to linux or even wayland)

This is caused by {DecryptVerfyFilesDialog, SignEncryptFilesWizard}::readConfig calling QWidget::winId, which creates a window handle for the widget if it is not a native window, which causes the broken behavior
(for reasons that I don't entirely know and that are not relevant here).

Both dialogs are actually native windows, so this should not happen. Apparently, at the point the function is called - at the beginning of the constructor of the dialogs - the window is not created yet, causing QWidget::winId to create one mistakenly.
We work around this by delaying the invocation of readConfig until after the constructor has finished