More virtualization testing
Made asset bundle detection less roundabout
Rework in progress; Added support for multiple "tags" instead of just "platform". Changed AssetBundleContents/AssetBundleListing to use object instead of string as primary key.
Moved AssetBundleHelperSettings out of Editor so AssetBundleListing can get the list of existing tags.
Moved GetLastWriteTime out of AssetBundleHelperSettings and into AssetBundleManagerWindow because LastWriteTimeUtc doesn't exist in Webplayer.
Moved BundleTag, BundleTagGroup, and PlatformTag out of AssetBundleHelperSettings.cs and into their own files.
Added an id field to AssetBundleContents.
Made AssetBundleContents.LoadOrCreate() recreate expected but missing files.
Made AssetBundleListingEditor read/write AssetBundleContents properly and switch between tag groups without freaking out.
Fixed AssetBundleListingEditor.TagComboBuilder entering an infinite loop if given an index >= list count.
Changed ListingEditorEntry to look up the default asset using a specified tagString instead of having a separate default asset reference.