remoteSearch: Load remote search providers synchronously

remoteSearch: Load remote search providers synchronously

As we only reload search providers on startup or when the sort order changes,
and given the small number of search providers we'll actually load, I doubt
we'll see any speed decrease.

The simplicity of synchronous code is also much clearer, and fully avoids
all the possible bugs about in-flight requests or similar.

This also prevents issues with multiple search providers showing up at once,
which happen when multiple requests to reload search providers get called
immediately, with the existing in-flight async requests never cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=700283