Microsoft-ui-xaml: Refactor MajorSize and other helpers to make them easier to read and understand

[ranjeshj]

Describe the bug
See feedback in PR https://github.com/microsoft/microsoft-ui-xaml/pull/667
refactor the orientation based helper methods to make them easier to understand.

[chingucoding]

There are two/three ways we could improve the helpers:

• Rename them to GetMajorSize or GetPointerToMajorSize
• Provide a static SetMajorSize function that takes a float and a Size/Rect pointer
• Do both

What would be the best way to solve this @ranjeshj @StephenLPeters @jevansaks (because you were one of the people confronted with this during review) ?

[ranjeshj]

@jevansaks had a pretty good solution in the PR comments I think which we could run with.

float& MajorSize(winrt::Rect &rect)
{
    return isHorizontal ? rect.Width : rect.Height;
}

// use site:
MajorSize(bounds) = lineSize;

[chingucoding]

Seems like a good idea. I will create a PR for this soon if that's fine with you.

[ranjeshj]

Sounds good. Thanks!