particle: on-click: tilde expansion

We now do tilde expansion of the *first* argument in on-click
handlers.

That is:

  ~/bin/foobar.sh ~/arg1

is expanded to

  $HOME/bin/foobar.sh ~/arg1

(meaning, the handler will most likely *not* do what you’d expect)

Related to #307

particle.h

@@ -64,7 +64,7 @@ struct exposable {
 };
 
 struct particle *particle_common_new(
-    int left_margin, int right_margin, const char *on_click_templates[],
+    int left_margin, int right_margin, char *on_click_templates[],
     struct fcft_font *font, enum font_shaping font_shaping,
     pixman_color_t foreground, struct deco *deco);