AssetManager 0.5.5.54

Asset visual sources

Added a persistent per-asset display source setting.
Default display order is asset image, category image, MeshCentral icon, then placeholder.
Added explicit per-asset choices for asset image, category image, or MeshCentral icon.
Category images are now used consistently in the asset list as well as the asset detail view.
Category edit view now shows the current category image and allows clearing it.
Added documentation for the visual-source model and its relationship to MeshCentral field update rules.