refactor(images): unify handling of images

This commit unifies the way images may be retrieved by in the modeler.
From now on, all access to images MUST happen via the Images utility.

By using the utility

- image loading is delegated to Graphiti
- all registered IImageProvider_s (across different bundles) are taken
  into account for loading the image

Closes CAM-1499
chore(customTask): rename extension point definition

Related to CAM-1394
chore(diagram/features): restructure FeatureMap

Related to CAM-1394
fix(property-panel): refresh asynchronously

Related to CAM-1487
feat(customTask/palette): add palette integration api

It is possible to specify the palette integration for a custom service
task via the ICustomServiceTask#getPaletteIntegration() method.

Related to CAM-1394