Add PhotosSearchContext and make PhotosApplication implement it
We want to split the application into two search contexts - one is for searches initiated from within the application, and the other is for searches initiated from gnome-shell. For that to happen certain classes which were singleton, and hence global to the entire application, now has to have a separate instance for each context. Each context will, therefore, have a state that encapsulates the instances of these classes that correspond to it. Currently, PhotosApplication is the only context. The future PhotosShellSearchProvider will be another implementation.
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