[Applet]Update toolbar's checkbox spacing

Summary:
Move the switchButton's icon smallSpacing left.
Make icon size smallMedium.

BUG: 417710

Test Plan:
Before:
{F8108357, size =full}
After:
{F8108335, size =full}

Reviewers: #plasma, #vdg, jgrulich, ngraham

Reviewed By: #vdg, jgrulich, ngraham

Subscribers: broulik, plasma-devel

Tags: #plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D27453