applets/kickoff: add "Prefer symbolic icons in sidebar" checkbox

note: screenshots below are outdated, Application Launcher Settings changed but everything else looks the same

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Since commit 702841df kickoff tries to append -symbolic to caterory list icon names.

Breeze icon theme
-symbolic (kickoff looks like after 702841df)	not -symbolic (kickoff looks like before 702841df)

With Breeze icon theme both look good, but -symbolic looks bad with Papirus icon theme.

Papirus icon theme
-symbolic (kickoff looks like after 702841df)	not -symbolic (kickoff looks like before 702841df)

This merge request adds Prefer symbolic icons in category list checkbox (on (default) means -symbolic, off means not -symbolic) to Application Launcher Settings.