Launcher: Place "Empty cache" button in the help menu

[TMarc]

Since it is always a hassle to explain where the cache can be cleaned (Start the launcher, click on Configuration, click on Advanced, go to P2P tab, click on "Empty the cache" button), and this in different languages,
it would be much more easy and much more logical to place two new buttons in the Help menu of the Launcher:
"Empty the cache" and "Open Cache location".

This would also ease a lot the support.