mouse shortcuts: don't hardcode selpaste

Because selpaste is activated on release, a release flag was added to
mouse shortcuts which controls whether activation is on press/release,
and selpaste binding to button2 was moved to config.h .

button1 remains the only hardcoded mouse button - for selection + copy.

config.def.h

@@ -162,7 +162,8 @@ static uint forcemousemod = ShiftMask;
  * Beware that overloading Button1 will disable the selection.
  */
 static MouseShortcut mshortcuts[] = {
-	/* mask                 button   function        argument */
+	/* mask                 button   function        argument       release */
+	{ XK_ANY_MOD,           Button2, selpaste,       {.i = 0},      1 },
 	{ XK_ANY_MOD,           Button4, ttysend,        {.s = "\031"} },
 	{ XK_ANY_MOD,           Button5, ttysend,        {.s = "\005"} },
 };