make keyhandler abort key configurable via config.h

config.def.h

@@ -70,6 +70,9 @@ static const int THUMB_SIZE = 3;
 /* Following modifiers (NumLock | CapsLock) will be ignored when processing keybindings */
 static const int ignore_mask = Mod2Mask | LockMask;
 
+/* abort the keyhandler */
+static const KeySym keyhandler_abort = XK_Escape;
+
 /* keyboard mappings for image and thumbnail mode: */
 static const keymap_t keys[] = {
 	/* modifiers    key               function              argument */