Seamless file attachments and option to connect only to Tor onion services

[sgj3]

This PR makes the following changes:

1. The message composition 'Send' tab gets a new 'Attach File' button that launches a file picker and appends the base64-encoded contents of the chosen file onto the message being composed.
2. In HTML message view, clicking such a data blob link launches a file picker, and the base64-decoded contents of the data blob are written to the specified file.
3. The network settings tab gets a new checkbox which allows the user to restrict connection attempts only to hosts that end with '.onion' (i.e., onion services). This enables Bitmessage users to keep their Bitmessage network traffic within the Tor overlay network and completely avoid exit nodes in order to frustrate traffic analysis attacks.

See issues #1538 and #105

[sgj3]

@PeterSurda Any comments?

[g1itch]

Maybe split this into two PR's? That features are not related.

[PeterSurda]

For attachments, there is a reserved key in the extended encoding, but it isn't implemented. I would prefer that is implemented and finished, but it probably requires some alterations in the database: https://bitmessage.org/wiki/Extended_encoding