Complete dockable UI

This work is for beta branch to complete #518 / #3485:

• Make a new floating window when we "undock" a widget.
• Make possible to drop any customizable widget inside any dock / or create new dock inside
  • This should solve issues like https://steamcommunity.com/app/431730/eventcomments/599664623303524506?snr=2_9_100000_&ctp=2#c599664623303605901
• Make new "dock tabs" (probably new widget?) when a widget is stacked inside the center of other widget.
• Show proper feedback for these actions (dock at sides/as tab): For this we could replace DropzonePlaceholder with a generic DraggingFeedback class that contains the UILayer (no subclass of Widget); creating a temporal widget for feedback makes thing harder to update the UI/calculate hit points/drop position with current widgets. We should create a UILayer and show the whole feedback there depending on what widget we are going to drop in.
• Add UI notch/tab-like element to show hidden elements again #2299 / #5263
• Allow to customize other widgets like the ToolBox
• Support to dock user dialogs (Lua API Dialogs)
• #2844
• Review linked issues in #518 for any missed opportunity