js: Key object adjustments after discussion

• src/aKey.js changed fingerprint to setter (to avoid overwrites)
• src/gpgmejs_openpgpjs.js
  • Added a class GPGME_Key_openpgpmode, which allows for renaming and deviation from GPGME.
  • renamed classes *_openPGPCompatibility to *_openpgpmode. They are not fully compatible, but only offer a subset of properties. Also, the name seems less clunky