[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