Cache all the conversation in the ListView

Keep all the delegates in memory while showing. It could be a problem
if the conversation grows _a lot_ but it has to grow a lot.

Possible ways to solve it:
- limit the log lines that we'll be displaying. We're talking about a
plasmoid anyway. If the user wants to read the log, he'll use ktp-log-viewer.
- set interactive to false. Then we can scroll as much we want, but not with
the scrollbar
- buy a better aleix

REVIEW: 109051