Use a prefix key to execute the key handler

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

config.def.h

@@ -77,6 +77,12 @@ static const bool RENDER_WHITE_ALPHA = false;
 #endif
 #ifdef _MAPPINGS_CONFIG
 
+/* the prefix key: when pressed, the next key combo is passed to the external
+ * key handler
+ */
+#define PREFIX_KEYMASK ControlMask
+#define PREFIX_KEYSYM XK_x
+
 /* keyboard mappings for image and thumbnail mode: */
 static const keymap_t keys[] = {
 	/* modifiers    key               function              argument */