particle/progress-bar: allow user to configure an on-click handler

Since we're typically interrested in *where* (on the progress-bar) the
user clicked, we need a way to pass the clicked position to the
handler.

Normally, the on-click handler is expanded when a particle
instantiates its exposable. At this point, we (obviously) don't have
the click position.

This is solved by expanding the handler a second time, when the bar is
clicked.

Thus, the user can use the "{where}" tag in the click handler. "where"
will be expanded to a percentage value (0-100).

particles/progress_bar.h

@@ -5,4 +5,4 @@ struct particle * particle_progress_bar_new(
     const char *tag, int width,
     struct particle *start_marker, struct particle *end_marker,
     struct particle *fill, struct particle *empty, struct particle *indicator,
-    int left_margin, int right_margin);
+    int left_margin, int right_margin, const char *on_click_template);