search: Refactor providerIcon into providerInfo

In order to match the current mockups, the providerIcon
class needed to include both the name of the provider
and the label that informs the user about how many more
search results are available for that specific provider.
The latter replaces the plus sign icon that has been
used so far.

https://bugzilla.gnome.org/show_bug.cgi?id=749957