uSyncItemEventArgs.Cancel is reversed

[bachratyg]

Describe the bug
When handling uSyncService.ImportingItem setting e.Cancel=true actually allows processing the item and leaving on false skips the item. The exact reverse of what should happen based on the property name.

When the event is not hooked FireItemStartingEvent defaults to true. When the event is hooked and the handler is empty it defaults to false therefore skipping all items.

To Reproduce

1. Start from a vanilla umbraco+usync installation
2. hook the uSyncService.ImportingItem (see skeleton code below)
3. change something in usync folder and restart the app
4. the change will not be synced

This reproduces the bug:

public class USyncComposer : ComponentComposer<USyncComponent>
{
}

public class USyncComponent : IComponent
{
	public void Initialize()
	{
		uSyncService.ImportingItem += this.USyncService_ImportingItem;
	}
	public void Terminate()
	{
		uSyncService.ImportingItem -= this.USyncService_ImportingItem;
	}

	private void USyncService_ImportingItem(uSyncItemEventArgs<XElement> e)
	{
		if( condition )
		{
			e.Cancel = true;
		}
	}
}

This works around the bug:

// ...
	private void USyncService_ImportingItem(uSyncItemEventArgs<XElement> e)
	{
		e.Cancel = true;
		if( condition )
		{
			e.Cancel = !true;
		}
	}

Expected behavior
Setting e.Cancel to true should skip the item being imported.

About your Site (please complete the following information):

• Umbraco Version: 8.11.1
• uSync Version: 8.9.1 (content edition)
• Browser N/A

Additional context
I believe this line https://github.com/KevinJump/uSync8/blob/ca25f2eec51127b793d7b372380bcc6318b88141/uSync8.BackOffice/Services/uSyncService_Events.cs#L142 should be

return !e.Cancel;

Note that this would be a breaking change, no clue how to handle this without breaking the whole world.