Migrate app components to URLs that match their names

Currently, there are several components whose documentation URLs do not match their exact names. These components are:

• xfce4-notifyd
• xfce4-screenshooter
• xfce4-terminal

The following steps are needed to bring the documentation in line with the rest of the components

• Create new locations on docs.x.o to match their proper names.
• Update links on existing/old sub-pages to point to new locations.
• Remove outdated sub-pages
• Update links on 4.16 sub-tree in docs.x.o
• Migrate images to new location
• Update READMEs
• Update code pointing to help files in apps