Add InputGroupStyle and OutputGroupStyle to Node

[miroiu]

📝 Description of the Change

Add support for grouping connectors in Node.

Node properties:

• InputGroupStyle
• OutputGroupStyle

Example usage

<nodify:Node Input="{Binding Input, Converter={StaticResource GroupByShapeConverter}}"
             Output="{Binding Output, Converter={StaticResource GroupByShapeConverter}}"
             Header="{Binding Title}">
    <nodify:Node.InputGroupStyle>
        <GroupStyle />
    </nodify:Node.InputGroupStyle>
    <nodify:Node.OutputGroupStyle>
        <GroupStyle />
    </nodify:Node.OutputGroupStyle>
</nodify:Node>

Closes #144