Add search/filter bar

Summary:
This diff contains a much-requested feature: adding a search or filter bar to KMenuEdit's application tree view. The search bar automatically expands categories as you search, then un-expands when search is cleared.

FEATURE: 57314
FIXED-IN: 5.16.0

Before:

After:

Test Plan: KMenuEdit's codebase is a mess; I noticed that there was only one unit test run by ctest! I'd love to write more tests in the future, but for this feature we don't need much.

Reviewers: ngraham, Plasma, cfeck

Reviewed By: ngraham

Subscribers: ognarb, mlaurent, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18939