Plans: Make it possible to give plans a name.

(cherry picked from commit d9b553f130a6eda36e2eea217785f829d3d8624e)

src/saveload/plans_sl.cpp

@@ -21,6 +21,7 @@ static const SaveLoad _plan_desc[] = {
 	SLE_VAR(Plan, visible,        SLE_BOOL),
 	SLE_VAR(Plan, visible_by_all, SLE_BOOL),
 	SLE_VAR(Plan, creation_date,  SLE_INT32),
+	SLE_CONDSTDSTR_X(Plan, name, 0, 0, SL_MAX_VERSION, SlXvFeatureTest(XSLFTO_AND, XSLFI_ENH_VIEWPORT_PLANS, 3)),
 	SLE_END()
 };