Moved MetroFlatButton to Controls.Button.Xaml.

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 just need to 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 FlatButtonStyle}"/>
    </Grid.Resources>
</Grid>
```

**Regarding the massive differences, I'm not sure why they happened.** 

All I did was:
+ cut the MetroFlatButton style out from FlatButton.xaml into Controls.Button.Xaml
+ fix the example project to use the standard way of referencing a style.