Flatpak Support

To theme [Flatpak](https://flatpak.org/) applications, themes must also be packaged for Flatpak. An example of this for GTK is [org.gtk.Gtk3theme.Yaru](https://github.com/flathub/org.gtk.Gtk3theme.Yaru). An example of this for KDE is [org.kde.KStyle.Adwaita](https://github.com/flathub/org.kde.KStyle.Adwaita).

For GTK, I am currently using [pakitheme](https://github.com/refi64/pakitheme/) to automatically package Ant-Dracula, but there is also [this](https://github.com/tryton-vanmeer/Flatpak-Gtk3themes/tree/master/org.gtk.Gtk3theme.Ant-Dracula) user repository which has a manifest for an older version of the theme.