Potential crash with NSOpenPanel in BasePathEditorControl on Catalina

[sevoku]

With Catalina NSSavePanel panels run out-of-proc so that instantiation is not supported anymore (in fact it seems that it was never officially supported) and will crash, hence we have to use the static shared panels instead (NSSavePanel.SavePanel or NSOpenPanel.OpenPanel).

For more details see: dotnet/macios#6474

Affected code:

Xamarin.PropertyEditing/Xamarin.PropertyEditing.Mac/Controls/BasePathEditorControl.cs

Lines 43 to 49 in 38105d0

	this.panel = new NSOpenPanel {
	AllowsMultipleSelection = false,
	CanCreateDirectories = true,
	ShowsHiddenFiles = false,
	ShowsResizeIndicator = true,
	TreatsFilePackagesAsDirectories = true,
	};

Bug filed for VS for Mac: https://dev.azure.com/devdiv/DevDiv/_workitems/edit/989519