Fixes #1785. Adds Pos.Align - Align sets of views horizontally or vertically

[tig]

Fixes

• Fixes Add Pos.Align to ease aligning subviews: Left, Right, Center, Justify  #1785

Proposed Changes/Todos

• Prototype & experiment
• Build generic/flexible justification engine that will work for both text and view layout
• Implement prototype Pos.Justify that automatically justifies views on the same X or Y axis with the same Justification setting
  • This works. See Generic scenario.
  • Note Dialog/Message box is broken.
  • It won't work for real because the automatic behavior is too limiting.
  • Add GroupID - Default is zero and zero gets auto behavior
• Refactor Justification class to be an instance class that can hold all the settings, calling Justification.Justify(...) static method
• Add PosJustification Scenario
• Justifier -> Add events
• Pos.Justify -> Cache location
• Update Dialog (and MessageBox)
• Remove old TextAlignment and `VerticalTextAlignment``enums.
• Rename "Justify" back to "Align"
• Other Pos/Dim -> Cache location/size?
• Unit Tests

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

[tig]

While this does not yet have sufficient unit tests, it is ready for review.

I'm specifically looking for feedback on the API.

The typical use-case where someone has a set of horizontal subviews they want justified (e.g. Dialog's buttons) is:

Button button1 = new () { Title = "Button 1", X = Pos.Justify(Justification.Center), Y = Pos.Bottom() };
Button button2 = new () { Title = "Button 2", X = Pos.Justify(Justification.Center), Y = Pos.Bottom() };
Button button3 = new () { Title = "Button 3", X = Pos.Justify(Justification.Center), Y = Pos.Bottom() };
Add (button1, button2, button3);

If there are multiple groups:

const int CHECKBOXGROUP = 1;
Checkbox cb1 = new () { Title = "Check 1", X = Pos.Justify(Justification.Justified, CHECKBOXGROUP), Y = 0};
Checkbox cb2 = new () { Title = "Check 2", X = Pos.Justify(Justification.Justified, CHECKBOXGROUP), Y = 0};
Checkbox cb3 = new () { Title = "Check 3", X = Pos.Justify(Justification.Justified, CHECKBOXGROUP), Y = 0};
Add (cb1, cb2, cb3);

const int BUTTONGROUP = 2;
Button button1 = new () { Title = "Button 1", X = Pos.Justify(Justification.Center, BUTTONGROUP), Y = Pos.Bottom() };
Button button2 = new () { Title = "Button 2", X = Pos.Justify(Justification.Center, BUTTONGROUP), Y = Pos.Bottom() };
Button button3 = new () { Title = "Button 3", X = Pos.Justify(Justification.Center, BUTTONGROUP), Y = Pos.Bottom() };
Add (button1, button2, button3);

I'm open for suggestions for other ways to let devs have multiple sets of views that are independently justified other than the somewhat clunky "GroupID" concept. I've tried multiple things, including trying to make it automatic based on the opposite dimension, but what I have now seems to be the cleanest.