Port to Qt6

[probonopd]

In order to get Qt6 applications (e.g., Qt Creator in 2024Q1 packages) to use our theme and work correctly, we need to build a version of QtPlugin for Qt6.

Issues:

• Needs https://www.freshports.org/x11/qt5-x11extras/ but there doesn't seem to be a qt6-x11extras. Looks like the Qt X11 Extras module was removed in Qt 6. Argh! This type of change causes extra work and (for me) takes the fun out of development. See https://doc.qt.io/qt-6/extras-changes-qt6.html#changes-to-qt-x11-extras for a suggested migration path. Also see Qt6 Removes X11Info from Qt5::X11Extras Zren/material-decoration#60
• https://www.freshports.org/devel/libqtxdg/ only contains qt5xdgiconloader but not qt6xdgiconloader
• sudo pkg install libdbusmenu-qt6 works but CMake can't find Qt6ThemeSupport

[probonopd]

Half-working workaround to get global menus working for Qt6 applications:

sudo pkg install plasma6-plasma-integration
export QT_QPA_PLATFORMTHEME=kde
qtcreator

This results in /usr/local/lib/qt6/plugins/platformthemes/KDEPlasmaPlatformTheme6.so being used, which has support for global menus.

Attention: As of 2024Q1, doing this REMOVES the kf5-baloo package, which in turn renders the Menu unusable! (Presumably because both the Qt5 and Qt6 based versions of /usr/local/bin/kwallet-query want to install into the same location. What a mess!)