Fix incapacity open *.cws files

Summary: File filter was incorrect (there wasn't space between '.cws' and '|' symbol), and didn't recognizing `*.cws` files, so there weren't possibility for users to open any saved files, if they could not specify the file name directly (availability of this feature depends of the particular 'Open File' dialog window).

Test Plan:
1) Write some worksheet (for example, with lua backend)
2) Save as worksheet with file extention `.cws` (for example `foo.cws`)
3) Try to open, using 'Open File' dialog.

Reviewers: #cantor, asemke

Reviewed By: #cantor, asemke

Subscribers: aacid, #cantor, #kde_edu

Tags: #kde_edu, #cantor

Differential Revision: https://phabricator.kde.org/D11528