Sub-charts cannot be attached to displays that inherit from defaults

Steps to reproduce:

- Set up a new view with a "Page" display.
- Configure the page display to be any line/bar chart, while making sure that the style settings affect "All displays".
- Create a new "Sub-chart" display.
- Try to make the Page display the parent chart. In the dropdown select list, only the "Master" display will be shown.

This problem is because we don't use the get_option() method that resolves settings that inherit from the default master display. This means that setting up sub-charts may be mysteriously difficult to set up, since attaching to the master display won't have any effect on the page version of the view.