Automagically detect the search platform (iOS or Mac)

The plugin will now check the active Xcode scheme and based on that append a search keyword to the query.

Results: Dash will only search the iOS docset when you have an iOS Scheme active or the Mac docset when you have a Mac Scheme active.

This also considers the case in which the user has edited the keyword for the iOS or Mac docset by reading Dash's preferences.