-- sqref is a block of selected cells containing the active_cell (normally is just == active_cell)
-- active cell is the selected vell in this selection/view/pane
-- inactive selections use the top-left cell as the active_cell
-- NOTE: It doesn't seem likely that these should be 'A1' in all cases (set to 'A1' because current tests demanded it). Purpose of this parameter needs clarification
-- NOTE: [xlnt::selection] needs appropriate ctor's once the purpose of the parameters can be clarified
-- additional tests to cover both variants of the set function
-- additional tests to cover hyper links on cells with non-string data
-- modified hyper link set function to handle cells with non-string data
-- fixed test behaviour for a cell with no value (value is set to the string form of the link. Behaviour copied from Excel 2010)
-- Resolves two test failures caused by a crash when setting the title to the existing value removing the sheet from the 'sheet_title_rel_id_map_'
-- added empty title check
