kicker: add option to make root level icons visible

Summary:
In file applets/kicker/package/contents/config/main.xml an option 'showIconsRootLevel' is mentioned. It has following description:
"Whether to show icons on the root level of the menu."

This change provides implementation for this option.

Test Plan:
Manual test:
1. Change menu to 'classic menu' (kicker), notice root level menu items have no icons
2. Open menu settings widget
3. Notice new checkbox 'Show icons on the root level of the menu'
4. Check that checkbox and hit 'Apply' button
5. Root menu items should now have icons similar which look similar to icons in 'modern menu' (kickoff)

Reviewers: hein, #plasma, #vdg, GB_2

Reviewed By: hein, #plasma, #vdg, GB_2

Subscribers: GB_2, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19829