Refactor: use enum types for event modifiers & types

Use enum types for event modifier and types,
`wmEventModifierFlag` & `wmEventType` respectively.

This helps with readability and avoids unintended mixing with other
types. To quiet GCC's `-Wswitch` warnings many `default` cases needed
to be added to switch statements on event types.

Ref !136759

source/blender/editors/space_node/node_view.cc

@@ -255,6 +255,9 @@ static wmOperatorStatus snode_bg_viewmove_modal(bContext *C, wmOperator *op, con
       op->customdata = nullptr;
 
       return OPERATOR_CANCELLED;
+    default: {
+      break;
+    }
   }
 
   return OPERATOR_RUNNING_MODAL;
@@ -687,6 +690,9 @@ static wmOperatorStatus sample_modal(bContext *C, wmOperator *op, const wmEvent
     case MOUSEMOVE:
       sample_apply(C, op, event);
       break;
+    default: {
+      break;
+    }
   }
 
   return OPERATOR_RUNNING_MODAL;