Save and restore the value of EquationInputArea when users scroll

[rudyhuyn]

The current implementation never saved/restored the value of EquationInputAreas when users scrolled the list and DataTemplate where recycled by the list virtualization

Description of the changes:

• Refactor MathRichEditBox and add a (real) dependency property (the existing one were used a regular property) we can use for bindings.
• Move submission detection from EquationInputArea to MathRichEditBox

Also:

• Because the submission detection is faster, other items don't get the focus when users press many times Enter.

How to test?

• Enter 4-5 equations
• Type Enter many times to add more than 16 'empty' equations
  Before: Because of the virtualization, new equations will be initialized with some of the first equations you typed (and vice-versa, the first equations will disappear if you scroll up)
  After: The equations won't be mixed up

How changes were validated:

• manually

[EriWong]

With your changes applied, I'm seeing a crash around focus when changing an existing equation.

Repro steps:
Set equation to x and submit it.
Change equation to 2x and resubmit.

result: "Focus cannot be moved while getting or losing focus."

[rudyhuyn]

Focus cannot be moved while getting or losing focus

I can't repro but I will try to catch this issue

[EriWong]

Looks good and works well.