Edit Padding after adding it

When we do a `Alt+Enter` and selec `Add Padding` the `Padding` widget is added to our widget tree, but the cursor remains at the end of `Padding(`. It would be more elegant if the cursor be at the `Padding` value itself, just like Live Template does.

This way the value would be already selected, hitting `Enter` would accept it, and typing any other number would overwrite it.