Change getDisplayTitle to getTitle

[GuySartorelli]

The Title field will be used by default to view links in gridfields, for example in a report for broken or orphaned links. It is also used as the default field for displaying in other contexts.

If the user opts to not include an explicit title (such as is allowed by #20), there will be no title to distinguish what the link is.

Renaming the Title db field to something else, and renaming getDisplayTitle to getTitle will mean the default title will be displayed whenever users have opted not to include their own title.

Acceptance criteria

• getDisplayTitle is renamed to getTitle
• The "Display Title" is shown in the following place:
  • When a link is rendered in the front end.
  • When a link is displayed in the initial LinkField (Not the form)

Notes

• In a future major release of framework, it might make sense to have a method on DataObject which gridfields and other contexts can call to get the title they should fall back on for displaying. This will mean it can be easily defined per model, without relying on specific db fields existing and having values.

PR

• ENH Rename and move Title field #207