[WIP] ToggleSwitch issues

from #460 

- [ ] Change `{DynamicResource AccentColorBrush}` [here](https://github.com/MahApps/MahApps.Metro/blob/master/MahApps.Metro/Themes/ToggleSwitch.xaml#L100) to `{TemplateBinding SwitchForeground}` so that you can recolour this at the ToggleSwitch level.
- [ ] Change `ToggleSwitch` style [here](https://github.com/MahApps/MahApps.Metro/blob/master/MahApps.Metro/Themes/ToggleSwitch.xaml#L136) to use `x:Key` so you can specify a different `SwitchForeground` value easily:

```
<Style x:Key="CustomToggleSwitch" TargetType="{x:Type Custom:ToggleSwitch}"
           BasedOn="{DynamicResource ToggleSwitchStyle}">
            <Setter Property="SwitchForeground" Value="{DynamicResource CustomToggleSwitchBrush}"/>
</Style>
```