Ensure tools are activated when shown, clean Tool implementation

[ReMinoer]

I fixed an issue on tools restored as hidden at launch but never activated at layout loading or when shown.

It's a good idea to not activate hidden tools at layout loading but we need to activate them when they are first shown in that case. IShell.ShowTool will now activate tools if they are not active, even if the tools was already registered in the IShell.Tools collection at layout loading.

I also added a IShell.RegisterTool method and also cleaned the Tool class implementation, especially properties setters.