• Makefile.am: Add new backend.
• configure.ac: Configure desktop file for Qt5 backend.
• doc/pinentry.texi: Add Qt6 backend to docs.
• m4/qt6.m4: New.
• qt/Makefile.am: Use Qt6 instead of Qt5.
• qt/capslock.cpp, qt/capslock_p.h, qt/capslock_unix.cpp:

Port capslock detection from KWayland to KGuiAddons.

• qt/pinentryconfirm.cpp: Adapt to changed function signature.
• qt5/*: Copy from qt/ and rename. --

The only noteworthy change is porting from KWayland to KGuiAddons for capslock detection, the rest is copy&paste, s/qt/qt5, s/qt5/qt6, etc.

compile with and without kguiaddons (needs https://invent.kde.org/frameworks/kguiaddons/-/merge_requests/113); check that qt5 and qt6 dialogs work, check that capslock detection on wayland/x11 works