This was being used to pass the X11 atoms all over the place, but I
realize this is no longer necessary - we can just pass them directly to
our RealConnection when creating it.
Seems some applications don't expect their popups to be unmapped without
their consent, so they make act strangely, but at least it doesn't
crash.
Fixes#117
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.
