Add Flex.Clear() method to remove all items

flex.go

@@ -101,6 +101,12 @@ func (f *Flex) RemoveItem(p Primitive) *Flex {
 	return f
 }
 
+// Clear removes all items from the flexbox
+func (f *Flex) Clear() *Flex {
+	f.items = nil
+	return f
+}
+
 // ResizeItem sets a new size for the item(s) with the given primitive. If there
 // are multiple Flex items with the same primitive, they will all receive the
 // same size. For details regarding the size parameters, see AddItem().