Flatpak build

[LeandroStanger]

Please, consider supporting Flatpak and publishing Texpander on Flathub.

[NicoWeio]

I wouldn't mind, but I don't think that's actually useful for Texpander. You'd still have to add a keyboard shortcut manually and installing xdotool/zenity/xsel doesn't seem too problematic to me.

[g33k247]

We need a fix for Fedora 31!

[NicoWeio]

Alright, there is one more problem. From my short research I learned that Flathub needs a special directory structure, so we'd have to create another repo/branch or do some updates to this one. @leehblue seems to be inactive, though.
Also, what about icons? Somebody would need to create one.