feat: new Antergos NeXT Minimal (Xfce) edition — branding, theme, and packaging

Introduces a lightweight Xfce-based variant of Antergos NeXT to work
around the 2 GB GitHub Releases limit for the full Plasma ISO.

New packages in antergos-packages:
- calamares-branding-antergos-next-minimal: Calamares branding variant
  that says "Minimal (Xfce)" with a notesqml page explaining why this
  edition exists (Plasma ISO exceeded GitHub's 2 GB limit).
- antergos-xfce-theme: GTK 2.0/3.0/3.20 + xfwm4 theme based on Numix,
  recolored to Antergos brand accent (#4A9EFF). Includes xfce-notify-4.0,
  metacity-1, openbox-3, and unity support directories.

packages.yaml updated with both new packages in the build list.

packages/antergos-xfce-theme/PKGBUILD

@@ -0,0 +1,17 @@
+# Maintainer: Celestia Ludenberg <ash8820@proton.me>
+
+pkgname=antergos-xfce-theme
+pkgver=1
+pkgrel=1
+pkgdesc="Antergos NeXT Xfwm4/GTK theme (based on Numix)"
+arch=('any')
+url="https://github.com/Antergos-NeXT/antergos-packages"
+license=('GPL')
+groups=('antergos-branding')
+depends=('gtk-engine-murrine')
+source=("Antergos-NeXT.tar.gz")
+sha256sums=('SKIP')
+
+package() {
+  tar -xzf "${srcdir}/Antergos-NeXT.tar.gz" -C "${pkgdir}/usr/share/themes/"
+}