main/postmarketos-ui-xfce4: Add further improvements

Improvements from xfce4-phone source:

• Add screenshot to README.md
• Enable screen locking
• Disable Desktop icons "Home", "File System", "Trash"
• Enable single-click on Desktop
• Enable single-click in Thunar
• Change clock format
• Set postmarketos wallpaper

Additional changes in postmarketos-ui-xfce4:

• Installing greybird-themes (which is already set as default theme in /etc/xdg/xfce4/xfconf/xfce-perchannel-xml/xsettings.xml).
• Changing /etc/skel/.config/autostart/onboard-autostart.desktop to slightly speed up keysboard startup (overriding startup-delay of /etc/xdg/autostart/onboard-autostart.desktop).
• Enabling scrollbars by /etc/profile.d/enable-scrollbars.sh, this is helpful where touch-scrolling doesn't work like e.g. in whiskermenu.

(And re-ordered network-manager-applet in APKBUILD _pmb_recommends section.)

Currently marked as draft because: MR xfce4-phone!7 (merged) needs to get merged and and release 0.4 to be set up. Thereafter I'll change the source in APKBUILD back to $pkgver.