config: do key binding collision detection after loading the conf

This allows the user to write, to swap two key bindings:

  [key-bindings]
  show-urls-launch=Control+Shift+r
  search-start=Control+Shift+u

instead of:

  [key-bindings]
  search-start=none
  show-urls-launch=Control+Shift+r
  search-start=Control+Shift+u

This should simplify the configuration for people who replace a lot of
the default key bindings.

This also simplifies the parsing somewhat, since we no longer has to
parse a key-binding into a temporary list, ensure it doesn’t have any
collisions, and then copy it into the actual key binding list.

_While_ parsing a key-binding, we still need a temporary array (at
least for the time being), but that temporary array is no longer
visible outside the parsing function that allocates it.

config.h

@@ -45,6 +45,10 @@ struct config_key_binding {
     struct config_key_modifiers modifiers;
     xkb_keysym_t sym;
     struct config_binding_pipe pipe;
+
+    /* For error messages in collision handling */
+    const char *path;
+    int lineno;
 };
 DEFINE_LIST(struct config_key_binding);