[kicker] Use arrow functions in signal handlers

Summary: Fixes warnings from qmllint about unqualified property access. It also makes the code more readable since it's clear which are the signal's arguments

Test Plan: Did kicker/dashboard things

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26409