Dolphin Services: Add search box

Summary:
You can now filter the list of installed services.
This is especially useful if you install additional service menus
from the kde store, because one instalaton can add a lot of services.
For instance the ReImage plugin.

Test Plan:
Compiles, search for a service menu.
Only services matching the query will show up.

Reviewers: ngraham, elvisangelaccio, meven

Reviewed By: meven

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D29375