Make separate flag for controlling the x & y axis title + Support padding for axis title

[billy-addepar]

This PR aims to do:

• Split the hasAxisTitles into hasXAxisTitle and hasYxisTitle to have more granular configuration for X & Y axis titles
• The current measurement for X axis title is not correct and does not take into account of the legend padding for axis title. This PR makes legendChartPadding to be dependent on height of axis titles. The legendChartPadding is move from scatter-chart.js into axis-titles.js mixin.

@jordansmith42 @philpee2 @embooglement

[thangdinh]

So this one is not backward compatible? i.e. if people are using ember-charts and this upgrade will break their existing code base where they set the hasAxisTitles. We probably should support backward compatibility here.

[embooglement]

I agree with Thang, if we can support backwards compatibility here we probably should, otherwise we'll have to do a major version bump.

[billy-addepar]

backward compatibility supported.

[billy-addepar]

@thangdinh @embooglement

[billy-addepar]

Comments addressed + tests added

[thangdinh]

what will happen if we have a long title?

[billy-addepar]

@xudaniel11work for the long title question

[billy-addepar]

Also @jordansmith42 for that question. It's not supported in master branch now.

[jordansmith42]

@thangdinh that's an edge case that we should look into but shouldn't block this, it seems very rare that a title would be longer than the entire chart width

[billy-addepar]

Comments addressed

[jordansmith42]

LGTM if all the comments have been addressed