[Enhancement] Add a converter to convert string to Brush

#### Summary
The ViewModel stores color strings, while WPF controls require Brush objects.

#### API Changes
StringToBrushConverter

#### Intended Use Case
```csharp
public string BgColor { get; set; } = "#FF2E86AB";
```
```xaml
<Border Background="{Binding BgColor, Converter={StaticResource StringToBrushConverter}}"
        Width="200" Height="100"/>
```
