osx: Set Menlo as default font

This moves the find_font function to the front ends since it depends on
pango and cairo

src/common/fe.h

@@ -180,5 +180,6 @@ void fe_tray_set_icon (feicon icon);
 void fe_tray_set_tooltip (const char *text);
 void fe_tray_set_balloon (const char *title, const char *text);
 void fe_open_chan_list (server *serv, char *filter, int do_refresh);
+const char *fe_get_default_font ();
 
 #endif