Introduce an UIXP for the image dialog search tabs

Currently, the document tree and upload tabs are hardcoded in ImageSelectorService.
This prevent extensions to contribute there own tabs. It is in particular useful to allow recent version of XS to propose new dialogs (for instance, a grid search with image preview).

Proposed UIXP name org.xwiki.contrib.ckeditor.pluginsImageSelector.
Proposed UIXs for existing tabs:

• document tree: org.xwiki.contrib.ckeditor.pluginsImageSelector.documentTree
• upload: {{org.xwiki.contrib.ckeditor.pluginsImageSelector.upload}}

Parameters:

• priority: Lower priority tabs are displayed first. The UIX with the lowest priority is opened by default.
• id: unique identifier of the selector tab
• title translation key