Implement local message logs

Merged David Faure requested to merge work/dfaure/channel_logs into master

This was my initial reason for preferring a Qt-client over the official one, but we had many other things to do first :-)

Since messages can be edited afterwards, and to avoid dupes when reconnecting and history is replayed, the local storage is a SQLITE database. The user can however export that DB as a text file, on demand, using the action "Display / Show Channel Log".

Maybe in a few years we'll have to ask for a start date to avoid exporting many years, if that takes too long; we'll see :)

If you think the format of the text export is similar to konversation logs, that's, hmm, pure coincidence :-)

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