Encrypt/Decrypt changes

[0xKheops]

• modify api methods return values
• simplify popup UI

Currently both encryptMessage and decryptMessage return a value with this shape :

{
    "id":null,
    "result":"0xabcdef......"
}

id is always null and should be removed. also on the decryptMessage method, we could return the hex-decoded string value, instead of the current hex encoded value. => need to wait for Sumi's aknowledgement before proceeding as this would be a breaking change.

On the UI side, since the objective is not only to encrypt/decrypt small messages, but also files, it seems necessary to not show the message that are to be encrypted/decrypted Tho, if the message is less than 100 (TBD) characters, we could assume it's human readable and show it the user.