custom charts: Add classes for describing a chart

custom charts: Add classes for describing a chart

The patch adds a chart data class that contains the
information about which series to plot. The chart
model class contains information about the chart (e.g.
chart type, logscale, etc).

Change-Id: I831bf4d695c52230dd5c9a2c20a4c226d95e32c0
Signed-off-by: Gabriel-Andrew Pollo-Guilbert <gabrielpolloguilbert@gmail.com>
Reviewed-on: https://git.eclipse.org/r/76910
Reviewed-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>