Add worksheet extension (.cws) to name of saved file, if user haven't added it youself.

Summary: It's a little uncomfortable, that when we save cantor worksheet file with name like 'foo' and saved name also have name 'foo', because we use file filter in open file dialog and our 'foo' file will be filtered. So I have added check, that if extension for saved worksheet file don't specify, then we append it.

Test Plan:
1) Create worksheet
2) Save it with name `foo` as Cantor worksheet
3) Check that the file have name `foo.cws`

1) Create worksheet
2) Save it with name `foo.cws` as Cantor worksheet
3) Check that the file have name `foo.cws`

Reviewers: #cantor, asemke

Reviewed By: #cantor, asemke

Subscribers: aacid, asemke, #cantor, #kde_edu

Tags: #kde_edu, #cantor

Differential Revision: https://phabricator.kde.org/D11701