Scrollable tabs, TabButton and HorizontalListView fixes

Tabs Fixes and improvements

BREAKING CHANGE

TabItem needs now to be inside the Tabs property of the TabHostView:

<tabs:TabHostView x:Name="TabHost"
                    Grid.Row="2"
                    BackgroundColor="White"
                    SelectedIndex="{Binding Source={x:Reference Switcher}, Path=SelectedIndex, Mode=TwoWay}"
                    ShadowType="Top"
                    TabType="Fixed">
    <tabs:TabHostView.Tabs>
        <tabs:BottomTabItem Style="{StaticResource BottomTabStyle}"
                            IconImageSource="house_96.png"
                            Label="{localization:Translate Tabs_Home}" />
        <tabs:BottomTabItem Style="{StaticResource BottomTabStyle}"
                            IconImageSource="list_96.png"
                            Label="{localization:Translate Tabs_List}" />
    ...

• Added Scrollable TabType
• Added TabButton (for circle button)

[Feature] Enable Tab visibility toggle

#47 Binding to IsVisible property of the TabItem wasn't working, should be fixed with this release.

HorizontalListView holiday fixes

List goes blank after reset event

#81 This fix make it possible to bind the HorizontalListView to the DynamicData.Binding.ObservableCollectionExtended collection.

How to prevent a ListView from scrolling to the beginning at the ItemsSource update?

#77 Will keep CurrentIndex after reloading the bound ItemsSource.

[Android] Can't set padding correctly

#64 Weird little padding added on android removed.

[Android] DragAndDropEndedCommand called before the item is reordered in the collection

#71

[Android] ObjectDisposedException thrown when adding or deleting items to/from ItemsSource

#72 Added some guards to protect from adapter disposal.

[iOS] NullException at iOS.Effects.iOSViewStyleEffect.OnDetached

#75