Built-in item encryption/decryption

Currently there is ItemEncrypt plugin which handles item encryption/decryption and passwords through GPG client. Big downsides are:
- user have to install GPG,
- CopyQ uses it's command line which may change (it isn't API).

More appropriate would be to use (and bundle with the app) a simple library (or two) with stable API to handle encryption/decryption and passwords.

Libs to try:
- [GPGME](https://www.gnupg.org/(es)/related_software/gpgme/index.html) ([docs](https://www.gnupg.org/documentation/manuals/gpgme/)) - needs to be compiled on Windows