Option to hide indicator

[Oliver Sauder]

Requested by Andrea Corbellini:
This branch adds a new GConf key (/diodon/show-indicator) to let you disable the application indicator. The reason for this change is that for people like me, who use just the keyboard shortcut to access the clipboard, the application indicator is useless and often annoying too.

Note that I've not added this option to the preferences window. That's because if you set this option by mistake and if you don't know what the keyboard shortcut is, Diodon is unreachable. Instead, having just the GConf key allow only power users to use this option (i.e.: users that can easily roll-back mistakes).

[Oliver Sauder (sao)]

I have just reckoned that the naming of the new key is not consistent with the others. Therefore I have renamed it to show_indicator.