Make "Key Manager" accessible

Hi,

it would be nice if we could access the "Key Manager" from the options menu or integrate it into the option dialog.

Currently, you have to go into the settings of a SFTP site to manage your keys.

I noticed this accessibility issue as well and it's on my TODO list. I a moment to look over the code and Its a one liner, so I will be including this in the next build.

The Key Manager will now be accessible from the main menu under Sites.