Describe the new feature

A new dialog for editing conditional formats. The dialog should
allow adding and removing conditional formats, changing the priority order
and editing foreground colour, background colour and filter condition.

Additional info

Please answer these questions before submitting your feature request.

Is your feature request related to an issue? Please include the issue number.

To pull request #1503.

Does this feature exist in another product or project? Please provide a link.

It's similar to the functionality found in some Spreadsheets.

Do you have a screenshot? Please add screenshots to help explain your idea.