import audio/midi at "session start by default

import audio/midi at "session start by default

Too many users report they import their first tracks and can't find them.

Previous default was "at file timestamp". If a file has no timestamp,
the import dialog shows "00:00:00:00" but in absence of a timestamp,
Editor::add_sources() first checks target tracks and adds add it after
the last region, then falls back to use preferred edit position as
insert point.

In the rare case where a file has timestamps it may show up a few hours
in the future.

conclusion: import-at-timestamp is not a good default.