main/postmarketos-ui-xfce4: xfce 4.20 adaptions and additional small changes (MR 5942)

Changelog of xfce4-phone 0.6.0:
- Update screenshot
- Remove separator on the left-hand side of the clock
- Increase font size of the clock
- Change order of panel plugins "pulseaudio" and "power-manager"
- Increase icon size of status tray in panel
- After Xfce 4.20 release remove outdated "backdrop" settings
- After Xfce 4.20 release set theme "Greybird" explicitely

[ci:skip-build]: already built successfully in CI

main/postmarketos-ui-xfce4/APKBUILD

 # Reference: https://postmarketos.org/uipkg
 # Maintainer: Jakko <jahau@rocketmail.com>
 pkgname=postmarketos-ui-xfce4
-pkgver=0.5.4
+pkgver=0.6.0
 pkgrel=0
 pkgdesc="(X11) Lightweight desktop (stylus recommended)"
 url="https://gitlab.postmarketos.org/postmarketOS/xfce4-phone"
@@ -91,7 +91,7 @@ openrc() {
 }
 
 sha512sums="
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+3658095eb68748a60f491838a4d11e7cc34b2dccd4eac5628928af6f34bf99e1c15581fb0c88918b57e07df64e50bf04025f269ab532a83e6d15a6aac3640b4f  xfce4-phone-0.6.0.tar.gz
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