Make cpu_load_percent segment threaded

interval=0.5 means that it will block for 0.5 seconds which is bad. With 
             threading it blocks only the separate thread, and it does not hold 
             GIL (uses regular time.sleep to wait) in this case which is fine.
interval=0.05 means that it will report almost random value.
interval=None means that (assuming psutil.cpu_percent is called only by this 
              segment) it will report CPU load percent measured between two 
              subsequent .cpu_load_percent calls or cpu_load_percent call and 
              module import. It is used for update method to get immediate 
              result in case update_first is True.
Make dummy cpu_load_percent also ThreadedSegment

It is the easiest way to make documentation identical when created on a system 
with and without psutil module and deduplicate docstrings
Add gradient for cpu_load_percent

Note: no changes to colorschemes: no cpu_load_percent in colorscheme
Merge remote-tracking branch 'zyx-i/threaded-cpu_load_percent' into develop