Add a way to create a history service in read only mode
Create the EphyCompletionModel with a history service and bookmarks objects

Instead of getting them from the global shell.
Make the history service work even when the database could not be opened

The history could be used by the search provider even before the ephy
config dir has been created, if epiphany has never been launched. In
such case, the methods will simply fail to get results, and the search
provider will only return the special case for searching in the web.
Move the search provider to its own binary

https://bugzilla.gnome.org/show_bug.cgi?id=711409
Remove the headless mode

We don't need it anymore now that the search provider is in its own
binary