postmarketos-ui-xfce4: various improvements

Changes:

• Added back the xfce4-terminal package
• Added the xfce4-pulseaudio-plugin (also pavucontrol and pulseaudio-utils)
• Removed unnecessary gtk+2.0 dependency
• Added pm-utils (maybe this should be added directly in postmarketos-base?)
• Edit default panel config to include the pulseaudio item (also reduce icon size)
• Reduce desktop switcher to 2 desktops instead of 4?
• Change the xfce4 menu icon with the postmarketOS logo

Requires https://gitlab.com/postmarketOS/xfce4-phone/merge_requests/5 to be merged, new tag, and then bump the package version to the tag version

Tested on qemu and pinetab

Fixes #368 (closed), #299 (closed)