STYLE: Simplify parameter passing

Refactor the logic in Slicerappstore_IndexController::categoriesAction
to simplify mapping parameter names as received from the caller to their
names as they are passed to Slicerpackages_ExtensionModel, allowing us
to remove the special-case ternary where specifying the param array key.

Also remove remaining superfluous uses of '$avalue' lambda.
ENH: Improve search behavior

Tweak key handler of search input to set up a delay before updating the
results, and also to do nothing if the search text has not changed. This
fixes the page refreshing on press of e.g. navigation keys (arrows,
home, end, etc.) and improves responsiveness by allowing several
characters to be entered before a refresh happens.