Prefer to find oktetapart by desktop file, not binary name

Summary:
Okteta >= 0.26 installs the oktetapart binary in the kf5/parts subdir
of the plugins, so the old check will not find it.
But that version also installs a desktop file again, so prefer to use that.

Test Plan:
Okteta KParts plugin is found also with devel version of what will be 0.26
in some weeks.

Reviewers: Krusader, nmel

Reviewed By: Krusader, nmel

Subscribers: nmel, yurchor

Differential Revision: https://phabricator.kde.org/D18881

(cherry picked from commit 796b33f0e6c8b8545f7eb3bba8f6eb6f373e41c8)