Optimize certification trust level management.

A private key should be editable to Unknown or Ultimate levels only.
A public key should be editable to all levels except Ultimate.
Undefined level is excluded for both public and private keys.

The public WTreeTableNode may list private keys if the user does not manage
these private keys. Inform of this state in the tool tip. The certification
trust level is excluded for any change.

Tools.h

@@ -43,6 +43,7 @@ public:
     static WString GetKeyStatus(const GpgME::Key& k);
     static WString GetUidStatus(const GpgME::UserID& uid);
     static WString GetSigStatus(const GpgME::UserID::Signature& sig);
+    static bool KeyHasSecret(const WString& fpr);
     
 private: