Use FlatButtonStyle without referencing an additional ResourceDict

See: #1210
## Previously:

```xml
<StackPanel.Resources>
                 <ResourceDictionary>
                     <ResourceDictionary.MergedDictionaries>
                         <ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/FlatButton.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
 </StackPanel.Resources>
```
You had to include an extra `FlatButton.xaml` resource dictionary to make all your buttons `FlatButtonStyle`.

## After Pull Request, You Can Also
define a default style for buttons that is based on the `MetroFlatButton` style in your existing dictionary / within the `.Resources` property of your control.

**This is consistent with how all the other buttons in the program are used**

```xml
<Grid>
    <Grid.Resources>
        <Style TargetType="{x:Type Button}" BasedOn="{StaticResource MetroFlatButtonStyle}"/>
    </Grid.Resources>
</Grid>
```

Backwards compatibility is maintained as well.