Convert from UserControl to CustomControl

Add the ability to override default template in designer without any access to the original one.

I was not able to find a way to implicitly use the style from resource dictionary `Generic.xaml` as default style. ThemeInfoAttribute class do not exist like in WPF. So we need to register the resource dictionary manually like : 
```XML
    <Page.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="ms-appx:///PanelsView/Theme/Generic.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Page.Resources>
```
Breaking change in VLC for WP : In the App.xmal.cs `OnLaunched` event, the Frame control is not directly available anymore. We need to delay the `Navigate` call (Maybe in `MainPageHome.OnLoaded()` ).

Quick fix : add this line in 'LaunchTheApp()` method
```c#
await Task.Delay(200);
```