Be able to export/register menuitem icon directly from an addons

- Addons can exports custom icons, and clients (web / gtk) get thoses icons directly from server
  => addons can register custom icon as a "default" icon. (so replacing default one)
  => addons can register new icons
- Maintain a cache client-side, so icons don't need to be fetched each time
  (On connection, client ask for modification time to server,
   only update icons if modification time is newer that last time of fetch)

Use Cases:
   - You create a module for your client, and he want to display a logo for his company on some TLM (Top-Level-Menuitem) which contains all his specific menu.
   - You wan't to push custom icons themes directly from the server.
     All clients gtk (*nix and windows), client web; displays the same icons theme. (easier for user to switch between platform, between openobject clients)