XQuartz: pbproxy: The greedy CLIPBOARD handling now works for text.

This change adds some [self own_clipboard] calls in the necessary places to get the proper greedy behavior.

UTF8_STRING and STRING properties seem to work well now with the test cases (PRIMARY, and CLIPBOARD).  I can copy from several different X apps, and have the behavior be correct when pasting.  I also verified that quartz-wm isn't doing the copying, by disabling the quartz-wm paths.
(cherry picked from commit 934669f7)