[kicker] Never execute recent documents

Summary:
When one tried to open a recent document which happened to be
executable (a shell script, for example), it was run instead.
This was unexpected and could even be disastrous.

After this change, recent documents are always opened.

Reviewers: #plasma, hein, broulik

Reviewed By: #plasma, broulik

Subscribers: ngraham, hein, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10835