gui key management commands

[diveshuttam]

• backup key
• split key
• change password for key (egpg key pass)
• share
• renew
• revcert
• revoke
• delete

Note: the key generation(+ fetch, recover etc.) commands are offered at first usage when key is not found and not here. As currently using only one key as per egpg's current structure.

We may need to include other commands here if we use multiple keys in future.

[dashohoxha]

We may need to include other commands here if we use multiple keys in future.

There are no plans to support multiple keys in the same context.
But there are plans to support more than one context: #72