FindAsync and FindAllAsync

[linkdotnet]

Short meeting minutes from our call @egil

• Instead of Find and WaitForElement, we introduce FindAsync that behaves sync if a component is already there or async and like WaitForElement if it isn't (also we have to rephrase some xmldocs on BunitContext)
• Instead of FindAll and WaitForElements, we introduce FindAllAsync (with the same as above).
• Mark Find/FindAll as Obsolete
• Remove IRefreshableElementCollection and return IList for FindAll (anyway obsolete so no big deal)

[egil]

Motivation for this change: users can just use FindAsync or FindAllAsync and if the element is already there, it is returned immidiately. If it is not available, it will switch to "waiting mode", aka do what WaitForElement and WaitForElements does.

That makes users tests more stable. If they today have a test that finishes rendering synchronously, a test that uses Find will pass. However, if the component is refactored or changed to do async work before rendering the markup found by Find, then the test fails.

Whether a component is rendering synchronously or asynchronously is an implementation detail that we should help our users avoid leaking into their tests. This solves it with respect to working with the rendered DOM tree.