Add icons to Categories, Installed, and Sources globaldrawer entries

Summary:
At some point, icons were removed from a number of entries in the global
drawer, but with the many other changes which have happened since then
this now leaves the sidebar looking somewhere between overly vacant and
broken (with some overweight toward the latter). This patch adds these
icons back in, as well as a couple of others, which results in a global
drawer which looks considerably more completely loaded.

This is furthermore based on feedback from the Plasma Mobile Todo[1] notes
entry, which at time of writing contains the following (amongst others):

Discover:
Global toolbar:
- Installed and Sources should have icons
- Categories should either have icons or be less indented

[1] https://notes.kde.org/p/plasma-mobile-todo

Parent Categories created by the category reader

Test Plan:
See before without icons looks empty and just a tiny bit broken, and after
with icons included looks fully populated (screenshots incoming)

Before, no icons on the front page:
{F7090565}

Before, no icons in the applications subcategory:
{F7090567}

After, icons on the front page:
{F7090570}

After, icons in the applications subcategory:
{F7090572}

Reviewers: #discover_software_store, #vdg, ngraham, apol

Reviewed By: #discover_software_store, #vdg, ngraham, apol

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22716