Before, every time the clipboard selection changed, satellite would copy
everything on it and force itself to be the clipboard owner, regardless
of X11 or Wayland. Now, satellite is only the owner when the clipboard
owner comes from Wayland, and uses the XFixes extension to watch for
changes in clipboard ownership X11 side. Satellite also now avoids
copying all of the clipboard contents into memory every time, instead
copying directly on request. This is a pretty big change, but should
hopefully help make the clipboard more stable.
Also added some misc test cleanup/using helper functions where possible.
Using the XFixes extension may also end up being necessary for
implementing drag and drop, so it's good the infrastructure is there
now.
The ICCCM claims this is a "required" target for selection owners,
however several GTK clients (zenity, winecfg) don't seem to support it.
(So much for required.) Just manually grab all the supported targets
individually from the selection owner instead.
Fix for #50
