[runners/recentdocuments] disable executables or .desktop files

Summary:
It's possible to have a .desktop file in your recent documents list as
you were editing it. Either as a .desktop file or masquerading as
something else.

By default we would process the .desktop file like a .desktop file.

You do get a prompt if the .desktop file is not executable like in
dolphin.

Given we know from context that we're showing recent "Documents" we may
as well turn that behaviour off without risk of ill effects.

BUG: 419310

Test Plan:
Created .desktop file (masquerading as something else)
Had it in my recent documents after opening in another format
Loaded the file from krunner. It now opened in my text editor instead of running
the Exec line

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28369