X.A.WindowNavigation: better handling of floating windows and Full layout

[Rogach]

Description

Previous version of WindowNavigation just got "stuck" on floating windows, switching back and forth between floating window and tiled window underneath. This was because "magic point" was left in the same position and thus next navigation commands selected the same windows over and over again. Now the "magic point" is moved around such that it doesn't overlap with the previously selected window, and there are more complicated rules to make navigation between floating and tiled windows more natural.

The original behavior of navigating between tiled windows is preserved almost precisely (the tree-like layouts - which should be overwhelmingly the most popular ones - will have exactly the same behavior, the only differences might be in some convoluted custom layouts).

Previous version of WindowNavigation also prevented switching windows using focusUp/focusDown when in Full layout. Now there's a special case that handles such situations.

Checklist

• [x] I've read CONTRIBUTING.md
• [x] I've tested the changes with unit tests. Just look how pretty the tests are - there are diagrams that explain what each case is actually testing!
• [x] I updated the CHANGES.md file