Flatpak GTK3 themes

[Kyuyrii]

Added some commands to install all Flatpak versions of GTK3 themes used by Ubuntu Budgie.

[fossfreedom]

As per the last team meeting notes, the preference here is to update the wording of the flatpak description to say more about what happens when the user clicks the install button/buttons.

The preference of the wider team is to have two buttons - one to install the basic flatpak and one to install flatpak + the extra flatpak themes.

In the longer term we should look to add to the themes page to accommodate snap themes and flatpak themes - both to install & connect as well as uninstall.

[Kyuyrii]

As per the last team meeting notes, the preference here is to update the wording of the flatpak description to say more about what happens when the user clicks the install button/buttons.

There's no point in having a button to install Flatpak versions if some will be downloaded automatically when the user downloads Flatpak apps. That's how Flatpak works.

Not installing everything at once will only cause confusion.

Imagine that the user downloaded a Flatpak using the Pocillo-dark theme, the Flatpak theme came with it, then he switches to the Pocillo-light or Pocillo-slim-dark theme, and then the apps stop using the system theme. He probably won't know what's going on.