Added the ability to set the active sheet

Closes #653

include/xlnt/workbook/workbook.hpp

@@ -203,6 +203,12 @@ public:
     /// </summary>
     worksheet active_sheet();
 
+    /// <summary>
+    /// Sets the worksheet that is determined to be active. An active
+    /// sheet is that which is initially shown by the spreadsheet editor.
+    /// </summary>
+    void active_sheet(std::size_t index);
+
     /// <summary>
     /// Returns the worksheet with the given name. This may throw an exception
     /// if the sheet isn't found. Use workbook::contains(const std::string &)

source/detail/serialization/xlsx_consumer.cpp

@@ -2037,6 +2037,7 @@ void xlsx_consumer::read_office_document(const std::string &content_type) // CT_
                 if (parser().attribute_present("activeTab"))
                 {
                     view.active_tab = parser().attribute<std::size_t>("activeTab");
+                    target_.d_->active_sheet_index_.set(view.active_tab.get());
                 }
 
                 target_.view(view);

source/workbook/workbook.cpp

@@ -743,6 +743,11 @@ worksheet workbook::active_sheet()
 {
     return sheet_by_index(d_->active_sheet_index_.is_set() ? d_->active_sheet_index_.get() : 0);
 }
+void workbook::active_sheet(std::size_t index)
+{
+    d_->active_sheet_index_.set(index);
+    d_->view_.get().active_tab = index;
+}
 
 bool workbook::has_named_range(const std::string &name) const
 {

tests/workbook/serialization_test_suite.cpp

@@ -72,6 +72,7 @@ public:
         register_test(test_Issue492_stream_empty_row);
         register_test(test_Issue503_external_link_load);
         register_test(test_formatting);
+        register_test(test_active_sheet);
     }
 
     bool workbook_matches_file(xlnt::workbook &wb, const xlnt::path &file)
@@ -800,6 +801,13 @@ public:
         assert_run(rt.runs()[10], " first line ", "Calibri", xlnt::theme_color(1), 12, true, false, xlnt::font::underline_style::none);
         assert_run(rt.runs()[11], "Bold And Underline", "Calibri (Body)", xlnt::theme_color(1), 12, true, false, xlnt::font::underline_style::single);
     }
+
+    void test_active_sheet()
+    {
+        xlnt::workbook wb;
+        wb.load(path_helper::test_file("20_active_sheet.xlsx"));
+        xlnt_assert_equals(wb.active_sheet(), wb[2]);
+    }
 };
 
 static serialization_test_suite x;

tests/workbook/workbook_test_suite.cpp

@@ -77,6 +77,11 @@ public:
     {
         xlnt::workbook wb;
         xlnt_assert_equals(wb.active_sheet(), wb[0]);
+        
+        wb.create_sheet();
+        wb.create_sheet();
+        wb.active_sheet(2);
+        xlnt_assert_equals(wb.active_sheet(), wb[2]);
     }
 
     void test_create_sheet()