PrecacheAsync on ImageCache (#275)
PrecacheAsync on ImageCache (#275)

* Added PrecacheAsync method to ImageCache

* Fixed lock on wrong object.

* Changed PrecacheAsync to PreCacheAsync. Fixed typo.

* Combine methods for "get from cache" and "pre cache (assure)"

* Consistent naming for PreCache feature.

* GetFromCacheOrDownloadAsync now returns image if it is found in the memory cache even if "PreCache" is requested.
GetOrPreCacheAsync now creates a new "Get task" only if current "PreCache" task did't return an image.

bkaankose/UWPCommunityToolkit,phmatray/UWPCommunityToolkit,bkaankose/UWPCommunityToolkit,phmatray/UWPCommunityToolkit