Use "wxWindows licence" and not "wxWidgets licence" (the latter doesn't
exist) and consistently spell "licence" using British spelling.
See #12165.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64940 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
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
