bf6dae2151
The comment suggested Get{First,Next,Prev,Last}Line() should be changed
to const functions once compilers understand the 'mutable' keyword.
Hopefully now, after 19 years, compilers do understand the 'mutable'
keyword, but regardless of that the functions should not be const, as
they very clearly modify the state of the object that is observable
through the public interface.
Closes https://github.com/wxWidgets/wxWidgets/pull/1750