Summary:
The most serious change from this patch is to move the asynchronous replying to a request from the app for more messages to a newly-spawned, self-destructing thread. Within that thread, we block until the remote device replies with the requested messages.

All gotten messages are cached in the ConversationDbusInterface, so all future requests are fast and don't hit the remote device.

Test Plan: After applying this diff, the messaging app should show 10 messages every time it is opened

Reviewers: #kde_connect, nicolasfella, albertvaka

Reviewed By: #kde_connect, albertvaka

Subscribers: albertvaka, apol, nicolasfella, kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D16475