Add KeepCenterOnSizeChanged attached property.#818

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GF-Huang wants to merge 3 commits intoHandyOrg:masterfrom

GF-Huang:Add-KeepCenterOnSizeChanged

Commits on May 10, 2021

Add KeepCenterOnSizeChanged attached property.
GF-Huang
committed

Commits on May 21, 2021

Fix issue which typing in multi-line TextBox.
GF-Huang
committed

Commits on May 31, 2021

Consider user manually resizing.
GF-Huang
committed