Snapcraft PR: add icon and sound themes to KDE Neon

Applications using the KDE Neon extension don’t use the system cursor theme because the applications aren’t connected to gtk-common-themes. As a result, KDE Neon applications use a weird pixelated cursor on the default Ubuntu desktop.

Unlike its name implies, the cursor, icon and sound themes in gtk-common-themes are also applicable to Qt/KDE applications. This PR adds the plugs for icon (+cursor) and sound themes to the Neon extension.