Kexi/Plugins/Reports/Object tree

Design page for task: "Add object tree for the Report Designer".

Rationale

Kexi Form Designer has its object tree. The same feature for reports is needed.

Picture1: In many aspects the report's tree is similar to form's tree, so should be designed with code sharing and similarities in mind:

R1. Use the tab as in the form's tree, see the Picture1. The only difference that the tab's tooltip should say "Elements", not "Widgets". This difference should be used where applicable in other places.
R2. The tree must present hierarchy of all sections, then elements in each section. Example, for this report:

the tree should have the following items:

R3. Even invisible sections shall be displayed in the tree as items. Display them using semitransparent text color to indicate their invisible property.
R4. The tree should have two columns: Element name, Type
R5. Report itself is an (selectable) object and has properties, so it shall be a root of the tree.
R4. Each item in the tree should have a small icon, like the form's tree has. Icons are provided by the plugin(s) that provide the report elements creation (the Report Design toolbar uses them too).

TODO: Plan code sharing, refactoring here...

R3.1. Show/hide action for such tree items should be available for quick access. TODO: explain this.