Convert clipboard integration code from libX11 to xcb

This drops support for COMPOUND_TEXT.  Presumably some ancient
(pre-2000) clients exist which support that, but not UTF8_STRING, but we
don't have an example to test with. (Given the nature of the thing, the
users of those clients probably work in CJK languages)

Supporting COMPOUND_TEXT would also involve writing (or extracting from
Xlib) support for the ISO 2022 encoding.

v2:
Fix the length of text property set by a SelectionRequest

The length of the text property is not neccessarily the same as the
length of the clipboard text before it is d2u converted (specifically,
if that contains any '\r\n' sequences, it will be shorter as they are
now just '\n')