Prefer to find oktetapart by desktop file, not binary name
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.
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)