implement open_file_dialog, save_file_dialog and set_window_title for linux

src/work-calendar.cpp

@@ -215,6 +215,13 @@ void per_frame(){
                     if(save_file_path) free(save_file_path);
                     save_file_path = new_save_file_path;
                     save();
+
+                    char *title = (char *)calloc(1, strlen("Work Calendar - ") + strlen(save_file_path) + 1);
+                    memcpy(title, "Work Calendar - ", strlen("Work Calendar - "));
+                    memcpy(title + strlen("Work Calendar - "), save_file_path, strlen(save_file_path));
+
+                    set_window_title(title);
+                    free(title);
                 }
             }