[applets/kicker] Set icon for all applications group

Summary:
Most group have icons, this one did not. A new icon just got created for
all applications, it makes sense to be consistent.

This has no effect in the kicker-dash as that does not show icons, nor
kicker/kickoff as they don't show the all applications group.

Only third party menus will be affected.

Test Plan:
Looked at my custom menu
Looked at kicker-dash

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25294