Dark Drag'n'Drop'n'Refresh

Fixes

• #94 fixed background color and shadow type update for TabHostView
• #95 fixed UpdateTextVisibility throwing null reference exception while styling the global class
• fixed usage of RefreshView with HorizontalListView on ios
• fixed tintable image on ios with a lovely nasty trick
• disabled [android] RefreshView while drag and drop is in progress

Improvements

• added material dark theme to MaterialFrame: elevation is converted to dark background according to: Material dark theme specs
• #85 added IsDraggable property to DraggableViewCell to prevent the dragging of chosen items
• #91 added clicked event handler for TabButton
• #97 updated IconImageSource property type from string to ImageSource

Refactoring

• TaskLoaderView is now a package dependency
• NotifyTask has been replaced by TaskMonitor