Update smithay-clipboard to SCTK 0.9.1 (#16)

This commit entirely reworks the internal structure of the entire crate,
as well as some of its APIs. This crate only accepts a C pointer to a
Wayland display object, since the target audience of this crate are
libraries without a wayland-client types. Also since seat information is
not presented in such clients most of the time, the clipboard entirely
relies on its seat tracking.

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+- Minimal rust version was bumped to 1.40.0
+- Add support for `UTF8_STRING` mime type
+- Clipboard now works only with extern display
+- Clipboard now works only with last observed seats, instead of optionally accepting seat names
+
 ## 0.4.0 -- 2020-03-09
 
 - Fix crash when receiving non-utf8 data