Improve handling of invalid filenames and directory names

[khalim19]

Each platform (Windows, Unix, ...) has different sets of disallowed characters in filenames. So it would seem that deleting different invalid characters on each platform would be a good idea. However, many "exotic" characters on Unix platforms have apparently been more trouble than it's worth.

At the end of the day, it may be a good idea to leave the current allowed character set as-is to keep things simple.

On Windows, don't allow some "reserved" words to be in filenames (however unlikely it is that the user names the layers like that):
http://msdn.microsoft.com/en-us/library/aa365247%28VS.85%29

[khalim19]

Resolved by using StringValidator subclasses (for filenames, directories and file extensions) from the pylibgimpplugin library.