Add "Close project"

Allow the user to close a project from the project manager.
Distinguish the open project from the file history.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/4964

include/settings/app_settings.h

@@ -128,6 +128,8 @@ public:
     {
         bool                  first_run_shown;
         int                   max_undo_items;
+        std::vector<wxString> open_projects; // using a vector if in the future
+                                             // we want to open multiple projects at once.
         std::vector<wxString> file_history;
         int                   units;
     };