Prepare for a Plasma 6 release

So Plasma 6 is around the corner. While it mostly is backward-compatible with our themes, we need to migrate our tools and some theme parts to Qt6.

Mostly things noted to be not working:

• Calamares QML might need porting to Qt6
• SDDM Theme - Needs porting to Qt6
• Splash theme (light/dark) - Needs porting to Qt6
• Wallpapers - It seems metadata should be ported to a metadata.json format while we use metadata.desktop.
• Look-and-Feel - still have to figure out. My bet is a metadata format the same as wallpaper.
• New Plasma logic doesn't allow us to change panel's icons separately, we might need to make a thin XDG-compliant icon theme that inherits from Breeze but provide our icons (e.g. Manjaro logo instead of Plasma etc) - can be done later
• Konsole doesn't show nerd glyphs - https://bugs.kde.org/show_bug.cgi?id=478549
• Plasma 6 uses Wayland by default, it's still not preferable to switch our users to it due to number of issues with XWayland and Nvidia
• Emojis don't work, colored emojis can be enabled with https://gist.github.com/charveey/091b11ea554436d15c7fffcf01129a8a. Maybe we can use that configs by default? - looks like upstream Qt bug, not worth messing with that our side
• Not directly related to Plasma 6 but maybe we finally should implement some suggestions from https://community.kde.org/Distributions/Packaging_Recommendations like systemd timeout - just a thought, not a blocker at all
• GTK theme is not being hooked on first launch
• Install breeze5 for Qt5 application in Plasma 6