[KIconLoader] Adjust emblem border depending on icon size

Summary:
The current fixed margin of 2px on each side leads to overlapping
emblems and occlusion of large areas of the main icon.

Test Plan:

Before:
After:

Reviewers: Frameworks, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16442