Add "all applications" category icon

Summary:
We currently have no accepted "all apps" icon. This patch creates one, inspired by the
old "homerun" icon which seems to have tacitly been used for this purpose (Kickoff uses
a monochrome version of this icon, and it's been used for the "All about the apps"
initiative).

Test Plan:
Not actually used anywhere yet, but it's available for future uses of "all apps" in
list items/categories/buttons/etc.

Reviewers: VDG, broulik, ndavis

Reviewed By: VDG, ndavis

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25291