Add xref and yref to colorbars

[hannahker]

Summary: Add support for container-referenced positioning for plot colorbars. See demo CodePen here: https://codepen.io/hannahker/pen/eYPMpVV

API changes: yref and xref parameters are added to colorbars.

Expected behaviour:

• The colorbar may be positioned relative to the full container area if colorbar.yref or colorbar.xref are set to 'container'.
• If a colorbar has container-referenced positioning, then space will be added to the _fullLayout._reservedMargin accordingly, meaning that colorbars may be automatically positioned to avoid components such as automargined axes.
  • If xref='container', then margin space will be added to the _reservedMargin on the left or the right of a plot area (depending if x > 0.5)
  • If yref='container', then margin space will be added to the _reservedMargin on the top or bottom of a plot area (depending if y > 0.5).
  • If both xref='container' and yref='container', then margin spacing will be added to the top or bottom if orientation='v' and left or right if orientation='h'.

The defaults for yanchor, xanchor, y, and x have also been adjusted for cases where container-referencing is applied.

TODO:

• Constrain x and y to be within [0, 1] if container referencing is applied.

See #6589 for similar changes applied for legends.

[alexcjohnson]

💃 Well done 🎉

[archmoj]

💃