Add Qt6 D-Bus support to fix "klipper" clipboard clearing
Starting with Debian Trixie, when installing KDE Plasma as the desktop environment, you get Plasma version 6, which by default also installs the corresponding Qt6 libraries and programs (instead of their Qt5 counterparts).
As a result, the functionality of "pass" to clear Klipper’s clipboard history (silently) fails because "qdbus" is no longer available by default. This leads to security issues, especially on shared workstations.
This patch attempts to solve the problem as briefly and cleanly as possible by using "qdbus6" if it is available.