Add proper extension as needed in Save as and Export

Add proper extension as needed in Save as and Export

Introduce the new FileName method ensureExtension, which does the following:
* if the extension is already correct (in a case-insensitive way), do nothing.
* if it is not correct, add the extension to the file name.

This is different from changeExtension that will fail in a case where
the file contains dots, but not a real extension, like newfile2.1.

Use this new method in renameBuffer() and exportBufferAs().

Fixes bug #11008.