Fix ItemContainer SelectedBrush with custom BorderBrush

[marko1olo]

Fixes #70

Description of the Change

This updates the default ItemContainer template so selection, preview-selection, and pending-connection highlight brush changes are applied to the named border inside the control template instead of setting ItemContainer.BorderBrush from inherited style triggers.

That keeps a custom BorderBrush setter in a BasedOn ItemContainer style from overriding the selected visual state, while still allowing an explicitly configured SelectedBrush to control the selected border color.

Verification performed:

• dotnet build Nodify\Nodify.csproj -f net8-windows
• dotnet build Nodify\Nodify.csproj -f net9-windows --no-restore
• dotnet build Nodify\Nodify.csproj -f net10.0-windows --no-restore
• Temporary WPF smoke check: derived ItemContainer style with BorderBrush=Blue and SelectedBrush=Red rendered the inner border blue when unselected and red when selected.
• git diff --check

Possible Drawbacks

The selected, preview-selected, and highlight states now change the template border's BorderBrush rather than the effective ItemContainer.BorderBrush dependency property value. This preserves the visual behavior and avoids BasedOn style precedence issues, but code that read ItemContainer.BorderBrush while selected would now continue to see the configured base brush.

• dotnet build Examples\Nodify.Calculator\Nodify.Calculator.csproj -f net8-windows
• dotnet build Examples\Nodify.Playground\Nodify.Playground.csproj -f net8-windows
• dotnet build Examples\Nodify.Shapes\Nodify.Shapes.csproj -f net8-windows
• dotnet build Examples\Nodify.StateMachine\Nodify.StateMachine.csproj -f net8-windows

[miroiu]

This fix is a breaking change because it will require everyone who overrides the ItemContainer.Template to manually manage the border brush. This affects the Shapes and Workflow example applications.

[marko1olo]

Thanks for calling that out. I updated the branch so the default template still has the inner Border template triggers, but the style-level BorderBrush setters are restored as well.

That keeps derived ItemContainerStyle setters from overriding the default template's selected/highlight visuals, while custom ItemContainer.Template implementations such as the Shapes example can still bind to ItemContainer.BorderBrush and get the selected/preview/pending-connection state.

Local checks:

• dotnet build Nodify\Nodify.csproj -f net10.0-windows
• dotnet build Examples\Nodify.Shapes\Nodify.Shapes.csproj -f net8-windows
• dotnet build Examples\Nodify.Workflow\Nodify.Workflow.csproj -f net10.0-windows (passes with the existing _layer unused-field warning)
• git diff --check