Add calendar search provider

Add OC_Search_Result_Calendar and modify the calendar search provider to
use it. This also moves the search provider into the 'lib/search' folder
 to consolidate all search providers.
Add gallery search provider

Add OC_Search_Result_Gallery and modify the gallery search provider to
use it. This also moves the search provider into the 'lib/search' folder
to consolidate all search providers.
Change documentation