More for applications now displays a category list

More for applications now displays a category list

Rework the previously extant Application category code to display
in the expanded list.  Add a "Frequent" category which corresponds
to the most_used_apps, and selected by default.

Instead of adding the background and shadow as expanded items to
the results/details panes as fixed, we slave the background/shadow
sizes to the results using notify::allocation.

Also clean up the code for sizing the details pane, using a common
function which adjusts its x position in one place.