fix error bar trace ordering [fixes #338]

[etpinard]

@mdtusz

In brief

This PR:

• removes <g class="errorlayer"> from the cartesian plot framework.
• error bars nodes are now consider part of the <g class="trace">
• error bars nodes are placed below all other point nodes in scatter traces and above all other point nodes in bar/histogram traces.
• updates an old (wrong) image baseline and adds another image test

Important considerations

• The ErrorBars component is now required in the Scatter and Bar plot and style steps, instead of being called from the module-loop in Cartesian.plot.
• I chose to keep the ErrorBars.calc call in the main Plotly.plot routine. The error bar calc step needs to be in the positionAndAutorange block to properly update the axes ranges. Perhaps, we could make a setPositions in Scatter and have it call ErrorBars.calc ?

[mdtusz]

Clean and simple. I don't have anything to critique on this one!

💃