doc: Modify 'Import mapping file' icon and title for callstack view

doc: Modify 'Import mapping file' icon and title for callstack view

It also adds a link to source lookup for LTTng-UST traces taken with LTTng 2.8+
(because it does not always work automatically when not in the same computer as
the one the trace was taken on)

Change-Id: If4f191d3e9fa650f852b1a2bed6857cd6e716a74
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/81404
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>