tmf.ui: make CommonXLineChartViewer hide the X axis label by default.

tmf.ui: make CommonXLineChartViewer hide the X axis label by default.

The X axis label is always "Time" for the common line chart viewer, this
patch saves some vertical space by not displaying it.

The legend is moved by default to the bottom, this saves a lot of space
due to time alignment.

Change-Id: I965290de363d2196140e03e60f3abe69fe35efb0
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/70776
Reviewed-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>