ZeroKLobby.csproj: add SecureSaidLine.cs (line formatting) & SimpleEncryptionRSAandAES.cs

SecureSaidLine.cs: is regular SaidLine.cs but playername is green coloured to indicate encrypted message.
SimpleEncryptionRSAandAES.cs: is an instance of cryptographic service used by PrivateMessageControl.cs for encrypting message for each PM tab.

PrivateMessageControl.cs: add code to exchange secret key & decipher incoming encrypted PM.
ChatTab.cs: encrypt outgoing PM message (the one typed by user only, not automated message)

ContextMenus.cs: add option to start encryption in PM
ToolTabs.cs: minor, allow to dispose any encryption instance in a PM tab when closed