refactor: move I/O operations out of constructors

[NathanBaulch]

This is important because constructors cannot be made async. This PR moves I/O operations into static Create factory methods or instance Load methods that are called from OnLoaded view events.

[love-linger]

This PR will not be merged because I'm refactoring these code in feature/async

[NathanBaulch]

Sure. For what it's worth, this PR is a good transitional step towards async without actually introducing any async (which is why I pushed it separately).

[love-linger]

OnLoad will be triggered at every time this control is added to visual tree. For example: swithing between HISTORIES and LOCAL CHANGES page will trigger the conflict control multi-times.

[NathanBaulch]

Fair enough, I admit I'm not familiar with the lifecycle of UI components. Hopefully there's a more appropriate place to put this code outside of view model constructors.

[love-linger]

I completed the first refactoring of the feature/async branch.

I think we still need Tash.Run in constructors.