searchengine: Populate filesystem model in an idle

searchengine: Populate filesystem model in an idle

When starting a search over a very populated filesystem, it
is possible that typing the first chars will return a too
high number of results. Even though iterating through the
cursor is in itself very fast, extracting the GIO information
from those many files at once is not going to be as fast.

In order to increase interactivity (i.e. not make things
possibly sluggish) iterate the cursor in an idle function
and add search results to the filechooser model little by little.

If the user keeps typing (as it is likely will happen), there
will be better chances to cancel and proceed to the next
query timely. If not, the results will be there soon enough.