Consistent Alt+BackSpace behavior

The default config specifies BackSpace as "\177". The default behavior
should persist across modifier keys, commonly Mod1 (Alt or Meta) which
is widely used to delete a word on readline and text editors, notably
Emacs.

This will make Alt+BackSpace behaves as expected, i.e. sends "\033\177"
instead of "\033\010" as previous default behavior.

Signed-off-by: Christoph Lohmann <20h@r-36.net>

config.def.h

@@ -303,6 +303,7 @@ static Key key[] = {
 	{ XK_Delete,        XK_ANY_MOD,     "\033[P",       -1,    0,    0},
 	{ XK_Delete,        XK_ANY_MOD,     "\033[3~",      +1,    0,    0},
 	{ XK_BackSpace,     XK_NO_MOD,      "\177",          0,    0,    0},
+	{ XK_BackSpace,     Mod1Mask,       "\033\177",      0,    0,    0},
 	{ XK_Home,          ShiftMask,      "\033[2J",       0,   -1,    0},
 	{ XK_Home,          ShiftMask,      "\033[1;2H",     0,   +1,    0},
 	{ XK_Home,          XK_ANY_MOD,     "\033[H",        0,   -1,    0},