[DeedleFake]

Plain bindings that kawa needs for three fixes in DeedleFake/kawa#16. Each one is its own commit and wraps a single wlroots or libwayland call or event, with nothing added on top.

• XDGPopup.UnconstrainFromBox wraps wlr_xdg_popup_unconstrain_from_box. The box is in the coordinates of the popup's root toplevel surface, as wlroots expects. kawa calls it on a popup's initial commit so menus near a screen edge flip or slide back onto the screen instead of spilling off it.
• EventLoop.AddFd and EventSource.Remove wrap wl_event_loop_add_fd and wl_event_source_remove, plus the WL_EVENT_* mask constants. kawa uses them to get a pid from the goroutine that waits on a program started from New back onto the event loop thread through a pipe. It then removes that program's New box when the program exits without ever opening a window.
• Xwayland.SetSeat wraps wlr_xwayland_set_seat. Without a seat, wlroots logs not handling selection events: no seat assigned to xwayland and X clients get no clipboard. wlroots keeps the seat until Xwayland is ready, so kawa calls it right after creating Xwayland.
• Seat.OnRequestSetSelection, Seat.SetSelection, Seat.OnRequestSetPrimarySelection and Seat.SetPrimarySelection, plus DataSource and PrimarySelectionSource handles, wrap the seat's request_set_selection and request_set_primary_selection events and wlr_seat_set_selection and wlr_seat_set_primary_selection. Clients, Xwayland included, can only ask for a selection, and nothing changes until the compositor sets it. kawa never handled those requests, so no client could set the clipboard at all.

Verified with go build ./... and go vet ./... against wlroots 0.20.2, and by running kawa built against this branch in a nested session:

• popups near the right and bottom edges, and their submenus, now flip or slide back onto the screen;
• a New box disappears once the program exits;
• copying and pasting works between X and Wayland in both directions, for both the clipboard and the primary selection.