ime: move preedit state from terminal struct to the seat struct

This ensures different seat’s don’t step on each others IME pre-edit
state.

It also removes most dependencies on having a valid term pointer for
many IME operations.

We’re still not all the way, since we support disabling IME with a
private mode, which is per terminal, not seat.

Thus, we still require the seat to have keyboard focus on one of our
windows.

Closes #324. But note that *rendering* of multiple seat’s IME pre-edit
strings is still broken.

wayland.h

@@ -256,6 +256,15 @@ struct seat {
                 int32_t cursor_begin;
                 int32_t cursor_end;
             } pending;
+
+            wchar_t *text;
+            struct cell *cells;
+            int count;
+            struct {
+                bool hidden;
+                int start;  /* Cell index, inclusive */
+                int end;    /* Cell index, exclusive */
+            } cursor;
         } preedit;
 
         struct  {