Make wxList iterators conform to input iterator requirements

Define "pointer", "reference", "difference_type" and "iterator_category"
typedefs to ensure that wxList iterator classes are seen as iterators by
the standard library in C++11 and later, as otherwise standard container
template ctors taking iterators couldn't be used with them because
they're only available if input iterator requirements are satisfied.

This notably fixes creation of std::list from wxList iterators; add a
test which didn't compile before to show it.

docs/changes.txt

@@ -64,6 +64,14 @@ Changes in behaviour which may result in build errors
   wxGraphicsContext::CreatePen() continues to compile and work as before.
 
 
+3.1.2: (released 2018-??-??)
+----------------------------
+
+All:
+
+- Make wxList iterators conform to input iterator requirements.
+
+
 3.1.1: (released 2018-02-19)
 ----------------------------