Fix Dashboard hang when restoring uninstalled widgets

[krschau]

Summary of the pull request

PlaceWidget could cause a hang or crash if the widget was deleted and then we tried to update the position via SetPositionCustomStateAsync. This change prevents that crash by only updating state on real widgets, after they are all inserted. PlaceWidget also was a somewhat confusing insertion middleman, and this change removes it entirely and just uses InsertWidgetInPinnedWidgetsAsync instead.

References and relevant issues

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes Dashboard doesn't load if widget definitions were deleted #2252
• Tests added/passed
• Documentation updated