Updated logo
Update README.md

Add logo
changed libxml2 to libxml++ instead
Make notebook reorderable
Changed discard button to clear button to avoid Gtk warning. Response stays reject
Added documentation for section and scene
Added documentation for section and scene

Cherry picked documentation for section
Added structure saving. still needs scene content saving.
Added saving of scenes. next:save on tab close && file reorder
add checkboxes for roadmap.
add checkboxes for roadmap.
Update README.md
Update README.md
Added file and folder move in case of structure reordering
Made it impossible to have twice the same name in the same toc
Several minor changes
Refacored EditWindow into MainWindow.
Refactored ProjectModel and ProjectStructure into a single file and added documentation
Refactored StructureView into ProjectTreeView, documented and clarified class
Refactored StructureZone and StructureMenu into ProjectTreeZone