Adding categories to design

In this pull-request we are adding categories to design.

@Benglin, please review.

There are couple issues with subcategories panel. Planned to create separate pull-request for fixing them.  Additionally before first resizing of leftside panel all subcategories grouped in one long-long line. It is because in MeasureOverride / ArrangeOverride is passed big length value (10000 and more).
Since the ```LibraryWrapPanel``` is always 50 pixels smaller than the ```CategoryListView```, can we not do the following in ```ListView.ItemTemplate``` and remove the converter? This way the content of ```ListView``` will always have 50 pixels padding (if we want the padding on the right, we can switch the column, too):

```xml
<ListView.ItemTemplate>
    <DataTemplate>
        <Grid>
            <Grid.ColumnDefinition Width="50"></Grid.ColumnDefinition>
            <Grid.ColumnDefinition Width="*"></Grid.ColumnDefinition>
            <Expander Grid.Column="1" ... />
        </Grid>
    </DataTemplate>
</ListView.ItemTemplate>
```