Fixed building with Qt 6.10.

Also, some code improvements and small fixes.

Adapted to changes in private headers of Qt 6.9.

Bumped the version to 4.1.0.

• Fixed compilation with Qt ≥ 6.8.
• Added plasma-applications.menu to the fallback menu files.

The library is ported to Qt6, such that the Qt5-based version 3.12.0 can be installed alongside it for supporting Qt5-based apps. Also,

• Considered the EV XDG_CURRENT_DESKTOP as a colon-separated list.
• Fixed an FTBFS with BUILD_DEV_UTILS enabled.

Just corrected a typo and bumped the version.

File association is fixed with desktop entries that are symlinks.

• Correct (menu/panel/…) icons are guaranteed for newly installed apps on the fly.
• The priorities of desktop-directories folders are fixed.

For making it possible to have Qt5 and Qt6 installations of libqtxdg alongside each other in near future, two changes are made to this point release:

1. qtxdg-mat has been separated from libqtxdg and put into qtxdg-tools.
2. The installation of config files has been made optional.

Main changes:

• String literals are supported in Exec keys of desktop entries.
• Enhancements to qtxdg-mat.