Rewrite the Image Plugin as components

As denoted by mflorea in https://jira.xwiki.org/browse/XWIKI-14937?focusedCommentId=97124&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-97124, we currently lack a component-based implementation of some of the features provided by the Image plugin (xwiki-platform-image-plugin). Having this implementation could help us move the plugin out of xwiki-platform and into xwiki-contrib or xwiki-attic.