Spreadsheet: Add undo/redo implementation

The Spreadsheet application currently does not support undo/redo,
and with this update, we are starting the process of adding this feature
:-)

Additionally, the save dialog has been updated to use
GUI::MessageBox::ask_about_unsaved_changes() for system cohesity.

Spreadsheet: Add basic undo functinoality

The spreadsheet application now has basic support for undo. Testing of
this feature is limited, and may not work as intended yet.

Spreadsheet: Add callback when a cell's value is changed

In addition to the callback being added, this commit also exposes the
SheetModel class via a getter in SpreadSheetView.

Spreadsheet: Remove debug statements and use cell change callback

This commit uses the on_cell_data_change callback from within the
SheetModel class. This allows for us to push/pop changes to the undo
stack.

With this, we have basic Undo/Redo functionality :-)

Spreadsheet: Actually add window::set_modified

Spreadsheet: Const-correctness :-)

Spreadsheet: Reorder the edit menu actions

Userland/Applications/Spreadsheet/SpreadsheetView.h

@@ -103,6 +103,8 @@ public:
 
     void move_cursor(GUI::AbstractView::CursorMovement);
 
+    NonnullRefPtr<SheetModel> model() { return m_sheet_model; };
+
 private:
     virtual void hide_event(GUI::HideEvent&) override;
     virtual void show_event(GUI::ShowEvent&) override;