Brought Okteta plugin back to life

Summary:
The changes consist of ending porting to KF5 and porting to the new Okteta API.
Details:

• CmakeLists fixed (new library names)
• Includes fixed (now referencing to a okteta and kasten subdirectories is required)
• Renamed all Kasten2 to Kasten (Kasten2 was renamed to Kasten)
• Small changes to end porting to KF5
• Removed code for Kasten1, old "Kasten" and removed cheching of KASTEN_VERSION define (dropped support for very old Okteta version)
• Removed KASTEN_NAMESPACE define and oktetaglobal.h as this made it empty
• Changed from .desktop to .json (support for .dektop files of plugins in Kdevelop was dropped)

Test Plan:
Basic features seam to work.
It's hard for me to check it fully as I don't know about all of the futures.

Reviewers: kossebau, kfunk

Reviewed By: kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D817