Move Shift + Insert to shortcut

Shift + Insert is used like a hot key for paste the selection, so it is more
logical move it to shortcut array instead of having special code for it.
---
 config.def.h |    1 +
 st.c         |   13 +++----------
 2 files changed, 4 insertions(+), 10 deletions(-)

config.def.h

@@ -93,5 +93,6 @@ static Shortcut shortcuts[] = {
 	/* modifier		key		function	argument */
 	{ MODKEY|ShiftMask,	XK_Prior,	xzoom,		{.i = +1} },
 	{ MODKEY|ShiftMask,	XK_Next,	xzoom,		{.i = -1} },
+	{ ShiftMask,		XK_Insert,	selpaste,	{.i =  0} },
 };