Allow creating empty file in filesystem browser

Summary:
This commit allows user to create new empty files from filesystem
browser plugin. Filenames without extension are supported.

Menu items which produce such files were missing from Create New menu.
This is due to the way MIME type filter was configured. The filter
included files of type text/plain. However, empty files without
extension, for example, an empty file named "empty", do not belong to
class text/plain.

Both kmimetypefinder5 and xdg-mime utilities classify such a file as
application/x-zerosize. According to the current Shared MIME-Info
database, application/x-zerosize MIME type is not a subclass of type
text/plain, see
https://gitlab.freedesktop.org/xdg/shared-mime-info/blob/58bee0a8b52f4ced82c3ac227e688e24dc4259bf/freedesktop.org.xml.in#L3966.

This commit extends the filter accordingly.

Reviewers: #kate, cullmann

Reviewed By: #kate, cullmann

Subscribers: cullmann, dhaumann, ngraham, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D17764