Help contributors to help Sakia

Created by: galuel

• Propose in "preferences" menu a button to clear the cache folder, in order to simplify the operation for end users.
• Propose a help menu somewhere, with a link to a "how to" webpage (github for instance, or better sakia-wallet official site), where it is explained how to launch Sakia with log : ./sakia -d from terminal, or even better perhaps, propose a "with log" menu option (with restart), that will automatically create a log file (with folder address chosen by end user).