SearchActivity: String category -> int object_type
SearchActivity: use the new provider
SuggestionsProvider: add cursor's columns indices
SearchActivity: use constants from new provider
Remove the obsolete ContentProvider
strings.xml: remove obsolete strings

The names of the object types (categories) are now stored in the
object_types database table.
SearchActivity: use getApplicationContext()
Make a general Constants class

Move all the constants from DatabaseConstants to Constants.
SearchActivity: object_type -> objectType
SearchActivity: use ARK names instead of URLs
Move object types constants to Constants
Constants: add constants for URL building
Add DataServer to substitute BnfDataWebsite

Whereas BnfDataWebsite allowed to download the JSON data provided by
data.bnf.fr, DataServer allows to download custom data provided by the
server pointed by the JSON_OBJECT_URL_FORMAT constant.
Use DataServer and ARK names

Instead of BnfDataWebsite and URLs.
Remove the obsolete BnfDataWebsite
.gitignore: add assets/suggestions.jpg