If the user asks for clipboard data with neither wl_keyboard
nor wl_pointer focus being presented on a seat, we should ignore the
request and reply with error, instead of crashing.
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.
Previously we were always blocking for 50ms no matter what, but now
we'll be dynamically adjust our block ratio depending on the current
clipboard load. In the worst case scenario when user used the clipboard only
once will go with the next sequence: 0 1(16 times) 1 2 4 8 16 32 50 50
50 and so on, until the next user request.
