Make available in Flathub

[Gabriel-p]

Right now it's available as a snap but not as a flatpak app

[suve]

I spent a bit of time looking into this. I'm rather new to Flatpak, so I may be wrong, but it looks to me like Leafpad's dependency on intltool will be problematic, as it seems that's not available in either the Freedesktop nor Gnome SDK for Flatpak.