Search results in the history window to are scrolled to and highlighted
once the user clicks on one of them. Messages are searched by text in
the log database, but then looked up in the history text buffer via a
visible or invisible timestamp obtained from the database record.

Timestamps in the history text buffer, depending on user configuration,
may only have a coarse granularity. Philipp Hörist discovered that this
may lead to the highlighting of the wrong line in the history text view
and proposed an alternative solution that is implemented in this patch.

Timestamps are abolished as a means of searching messages. Instead, add
a mark with the unique message id (log line id) at the start of every
message in the history text buffer when the conversation is loaded.
After fetching the search results from the database this id can be used
to unambiguously find the corresponding message in the history text
buffer.