main/postmarketos-ui-xfce4: set background for login greeter (MR 5889)

Actually Xfce4 is capable to use the "draw-user-backgrounds" option of
"slick-greeter", applying the users wallpaper on the greeter.

However, as it turns out, the users background in pmOS Xfce4 is set at first
login of the user. Therefore, the greeters background is black at the first
boot.

Therefore set the greeters background to the default postmarketOS wallpaper.

main/postmarketos-ui-xfce4/10_slick-greeter.gschema.override

 [x.dm.slick-greeter]
+background="/usr/share/wallpapers/postmarketos.jpg"
+draw-user-backgrounds=false
 onscreen-keyboard=true
 onscreen-keyboard-layout="/usr/share/onboard/layouts/Phone.onboard"

main/postmarketos-ui-xfce4/APKBUILD

@@ -2,7 +2,7 @@
 # Maintainer: Jakko <jahau@rocketmail.com>
 pkgname=postmarketos-ui-xfce4
 pkgver=0.5.2
-pkgrel=8
+pkgrel=9
 pkgdesc="(X11) Lightweight desktop (stylus recommended)"
 url="https://gitlab.postmarketos.org/postmarketOS/xfce4-phone"
 arch="noarch"
@@ -91,7 +91,7 @@ openrc() {
 sha512sums="
 a30b34fb87150810a291faa71726822942fc2b49e418fe3011445f7921b54bdf645e52ff61f31f61fbe799f221931f1d1a17f04b7378c2d72f4eb1795e5d78b7  xfce4-phone-0.5.2.tar.gz
 114d77e6e7c11fc87244fc7c179244274c8f8711f111fcb1b9774ef6b226bd2bea01f85395da130f53670a870cfe571babd7f1c233ba85e1ef5a056ab65bd0b7  10_onboard.gschema.override
-28eb2bda33ad9c45afa915a88afbc56e2843e5b707b017f583eec22704fee5980a6f7b4a646151807dc6e8ed190b7dc2877f888ec0c36f1d8494d3be223c1f14  10_slick-greeter.gschema.override
+7ce4ee56bbff8052ceb14f52e010ae48290589f45dfd0afbb240bf95abac1bd02e7a75a22776f90a1c96b59286ec4d8b6cbe18881550f4488b589cbf2b12bfc8  10_slick-greeter.gschema.override
 c11a66b639570ecad51b97c127898afc3f123e2de95ea2a788abc51a45c23c82dbf1022224c15a88691656a8065874d6f396aee0df64e93e1966a82b32d6a31c  70-lightdm-pmos.conf
 262cdcf334cfa56872797c960cb8683b79c7b13b0a8179cd002e598576f0e9fe07057fd0fe724e87a6664db4b91b31a8c0cbf026ab137533b07675f838ae57bb  enable-scrollbars.sh
 ce7921f500b603796572bc02fa123e58d58ed1e9efd4d909de8f73c11a3e0bc0893639ce159f0701578b19db764893d021502d1c98cc24eb692e9d48320ff68c  onboard-autostart.desktop