Seat: avoid leaking eaten keys to client on focus

Until now, Seat.setFocusRaw sent all pressed keys to the client,
including ones that should be eaten. (Try e.g. changing focus to a
nested wlroots compositor with a terminal open to easily see it.)

However, only filtering out the eaten keys is not enough; they were
eaten only once all mappings had been executed. Therefore, the original
function had to be split into one looking up mappings and another
executing them.

river/KeycodeSet.zig

@@ -50,3 +50,8 @@ pub fn remove(self: *Self, old: u32) bool {
 
     return false;
 }
+
+/// Removes other's contents from self (if present)
+pub fn subtract(self: *Self, other: Self) void {
+    for (other.items[0..other.len]) |item| _ = self.remove(item);
+}