Increase the size of system tray icon hitboxes on the System Tray Plasmoid

Summary:
Depends on D27465

@ngraham recommended that I split up https://phabricator.kde.org/D27438 into three patches:

1. The foundation code to get landed first so that the other two patches can be done as patches that change two separate lines instead of just one

2. Tablet Mode increases tray icon spacing

3. This patch - Increase tray icon spacing slightly on Non-Tablet Mode (half of the amount it increases by in Tablet Mode)

This patch is for the third patch of the three recommended patches to split the original patch into, and makes the click areas for the System Tray Plasmoid slightly larger in normal Plasma.

Test Plan:
1. Install this patch to the System Tray Plasmoid

BEFORE:
{F8109485}

AFTER:
{F8109489}

Reviewers: #vdg, niccolove, ngraham

Reviewed By: #vdg, niccolove, ngraham

Subscribers: kmaterka, gvgeo, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27466