conversationsview: various bugs, modern C++, leaks

Fix a bunch of newly discovered memory leaks, port some code parts
using C-style fixed-length buffers to modern C++ and check call
status before displaying it as last interaction.

Change-Id: I6114585e80411717e89990653eff26bf7fc49de5
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>