[checkbox] Honor icon.[name|source] property
Like https://invent.kde.org/frameworks/plasma-framework/-/commit/b7ee6962501ffebcfb092f090c5adf7195df09ed, but for QQC2 checkboxes. Code change is significantly different since it's implemented differently here compared to there. End result is the same though:
[bomb icon in the KCM is just for testing purposes, obviously
cc @mart @ahiemstra