search-provider: avoid subprocess for search provider

Previously, this search provider would spawn gnome-calculator for nearly every keypress in GNOME Shell search. That means doing expensive things like not only spawning, loading dynamic libraries, but also setting up GSettings for the angle-units setting.

This brings that solve code over since the search provider is already linking against the calculator library. No subprocess is needed and it can reuse the settings appropriately.

Fixes #350 (closed)