Override some methods in wxF(F)ileStream to resolve ambiguities.

Override some methods in wxF(F)ileStream to resolve ambiguities.

Override virtual methods IsSeekable(), GetLength() and OnSysSeek/Tell() to
forward to wxF(F)InputStream base class as otherwise it's impossible to use
them at all because of ambiguity between the versions inherited from this
class and wxF(F)OutputStream (even though the two versions should do the same
thing as they operate on the same file descriptor/handle).

Also improve documentation of these classes: provide a brief description,
correct the base classes.

Closes #11577.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62995 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775