Generalize chart loading functions

* ag_app_import_chart() became ag_app_import_file(), and has the potential
  to load any file type
* ag_app_import_cb() so it can open multiple file types
* ag_chart_load_from_file() is now called ag_chart_load_from_agc()

src/ag-chart.h

@@ -48,8 +48,8 @@ AgChart *ag_chart_new_full(GsweTimestamp   *timestamp,
                            gdouble         altitude,
                            GsweHouseSystem house_system);
 
-AgChart *ag_chart_load_from_file(GFile  *file,
-                                 GError **err);
+AgChart *ag_chart_load_from_agc(GFile  *file,
+                                GError **err);
 
 AgChart *ag_chart_new_from_db_save(AgDbSave *save_data, GError **err);