Fix visibility bindings of undocked panels in specific cases

[scdmitryvodich]

Fixed two issues:

1. Undocked panel doesn't show after switching between tabs.
2. Undocked panel doesn't hide the first time it's unchecked from the main menu, while hiding/showing on the next attempts.

LayoutAnchorableFloatingWindowControl.cs
Added restoring of visibility binding on "activated" event.
Added possibility to disable/enable visibility bindings when docking manager gets unloaded ("unloaded" fires when switching between tabls).

DockingManager.cs
When "unloaded" fires, instead of closing all floating controls and creating them again when "loaded" fires, we transfer them to the list of hidden controls disabling their visibility bindings and event subscriptions. This solves issue 1).

LayoutAnchorableItem.cs
Added separate visibility reentrant flag for "LayoutAnchorable _anchorable". This solves case 2).

Also additional safety checks were added:
LayoutGridControl.cs - added checks for index bounds
LayoutAnchorableFloatingWindowControl.cs - added null reference check for _model.Descendents().OfType()

[scdmitryvodich]

Also were added unit tests for calculate and update of DocMinSizes and test which checks the state of visibility binding

[Dirkster99]

Thanks for thos unit tests. I'll be thinking about writing some tests as well, its just somewhat difficult here because many tests requires a view to be visible - please let me know if you see other 'obvious' areas where unit tests could be useful and I could take a look at that :-)