Optimize tabbed windows for practical purposes

[X512]

For now multitab windows are too heavy for everyday usage in applications. Every tab requires own client and server looper, drawing context etc. Also managing many multitab windows on workspace is difficult.

Possible performance and resource optimizations:

• Have one server looper and drawing context per multitab window
  
• Provide window-specific multitab handling. This means that window can handle tab list and provide active tab contents manually on client side.

Possible usage optimizations:

• Show window groups in Deskbar
  
• Make ability to close all windows in group
  
• Make ability to scroll or show invisible tabs in menu when there are a lot of tabs(like this was done in WebPositive)
  
• Make ability to save groups
  
• Make ability to tile groups
  

After this WebPositive and other native applications can be rewritten using SAT.

[pulkomandy]

I see a problem that you can stack windows from different apps using SAT. So running them all in the same looper does not sound possible. The other parts are good ideas but out of R1 scope.