Feature request: add the ability to open x-scheme-handler/flatpak+https

[fusion809]

I came across this post on Reddit. It seems to me like you could make Linux Mint that bit more beginner-friendly by making it so that x-scheme-handler/flatpak+https can be and is opened by mintinstall by default. I tried running:

xdg-mime default mintinstall.desktop x-scheme-handler/flatpak+https

in a terminal on a Linux Mint 22.2 Cinnamon live session and then a fresh install and while it does cause flatpak links to cause mintinstall to launch, the links are not actually opened in mintinstall. Hence why I assume this is a missing feature and hence a feature request is appropriate.

[fredcw]

cf #473