Currently, the only method available to set a Grid's Row and Column Definitions is via the adorners on-Stage, which poses some friction in terms of discoverability of a very important feature.

Ideally, the ability to add, remove, and edit the definitions of a Grid's Row and Columns, would be able to additionally be performed through the Grid's Properties Panel.

One thing to consider however is how to signal to the user that Grid Definitions are one of those Properties whose values cannot be keyed in Storyboards, Visual States, and Setters.
(Perhaps we grey them out out if in any of these contexts, with a tooltip on hover stating that the Definition values can't be keyed).