Move X-specific selection info into XSelection

Data about PRIMARY/CLIPBOARD and clicks are part of the front-end, not
the terminal.

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

st.h

@@ -149,12 +149,7 @@ typedef struct {
 		int x, y;
 	} nb, ne, ob, oe;
 
-	char *primary, *clipboard;
 	int alt;
-	struct timespec tclick1;
-	struct timespec tclick2;
-
-	//Atom xtarget;
 } Selection;
 
 typedef union {