Add key management functionality

[samuel-w]

This PR adds key management features of:

1. Password changing (Fixes Change password #303)

2. The ability to move the key from VortaKeyringDB to secure storage

3. Key import/export. (Fixes Import / path to repo keyfile #304, fixes Import / path to repo keyfile #803)

This issue I'm having is the input for 'borg key import --paper' happens in interactive console mode.
I would like to know if there is a way to take interactive console input into Qt, as I want to implement it without having to take parts of borg code.
My alternate solution is to remove paper key backup support.

Edit: Going to merge all the options into one tab. This also uses code from #550

Picture of UI:

[ThomasWaldmann]

Commenting on:

This issue I'm having is the input for 'borg key import --paper'
happens in interactive console mode.

I would like to know if there is a way to take interactive console input
into Qt, as I want to implement it without having to take parts of borg code.

My alternate solution is to remove paper key backup support.

I'ld suggest doing this PR without key restore capability for paper key hexdump data.

You could try adding that in a later PR (maybe after doing some ground work in borg to make it easier).

[samuel-w]

Closing to avoid running tests whenever I push. Will reopen later.

[samuel-w]

Marking the paper backup for later, the rest is fine.