REVIEW: 113720
DIGEST:

* Search installed Python plugins via trader
* Introduce `Kate/PythonPlugin` service type and .desktop files for plugins
* Make plugins management look-n-feel of C++ plugins management
* Enabled plugins now are per session (stored under "Enabled Plugins" key of the pate
config group), so it is possible to have different plugins in different sessions
(just like C++ plugins).
* Introduce per session configuration storage for python plugins (in addition to
a "global" one)
* Provide little more details if pate engine failed to load

... a lot of internal refactorings also have been done.