fix: add dbus interface to freedesktop secrets

[jnsgruk]

An alternative approach to solving #300, which is likely more secure, if it'll work!

To test this:

git clone https://github.com/jnsgruk/signal-desktop -b secrets-dbus
cd signal-desktop
snapcraft pack --verbose

# CAUTION: THIS WILL REMOVE YOUR HISTORY
sudo snap remove signal-desktop --purge

sudo snap install --dangerous ./signal-desktop*snap
sudo snap connect signal-desktop:dbus-secrets

Try to link Signal, then close it and re-open it.

[jnsgruk]

@vgallissot are you willing to give this a try?

[jnsgruk]

I've misunderstood how this works - we'll need to fall back to the password store idea.