Modified the return value of bobura::diagram_picture_box::release_mouse_capture().
Modified stdlib_headers.txt.
Corrected #includes.
Skipped a compilation error.
Added a TODO item.
Created bobura::view::diagram::item::p_item_by_position().
Created bobura::diagram_view::p_item_by_position().
Implemented the test cases.
Modified the constness of bobura::diagram_view:: and bobura::view::diagram::item::p_item_by_position().
Created bobura::diagram_view::unselect_all_items().
Created bobura::view::diagram::item::selected() and set_selected().