qt5.12: Add patches for QTBUG-73459 and QTBUG-69077

QT 5.12 introduced a regression, where a QT program wouldn't show its
tray icon, if there was no tray bar during program startup. (QTBUG-73459)

QT 5.12 introduced a regression, where qtwebengine applications would
freeze in some wayland compositors if a surface from the instance was not
visible (for example having a qutebrowser window on another workspace in
sway would freeze all qutebrowser windows).

Both got fixed already in Qt 5.12.4, but according to #57042 and its
sibling issues/PRs it doesn't seem to get fixed in near future for
nixpkgs.