engine: route popup preedit by client capability

dat.h

@@ -138,6 +138,10 @@ struct Drawcmd
 typedef struct Keyreq Keyreq;
 typedef struct Keyres Keyres;
 enum
+{
+	Cclientpreedit	= 1<<0,
+};
+enum
 {
 	Keypress,
 	Keyreset,
@@ -154,6 +158,7 @@ struct Keyres
 struct Keyreq
 {
 	void	*owner;	/* stable until the context's release is acknowledged */
+	int	cap;
 	int	op;
 	u32int	ks;
 	u32int	mod;