Every time I open/move the "Piano Roll" it's "fitted/pinned" to the project. How to disable this annoying feature by default?

prv is docked,press D to make it disapear or grab the handles at the top to resize it.
press X to close it from that screenset(down the bottom where it says pianoroll on the tab

All I want is to set the windows as un-docked by default. Is it possible?

You might have better response in the X Forum. This is for 8.5 and older. 

something is always going to be in the dock if you want to use anything other than track view.
track view is like the main view.
all those other views,console,prv,staff,exc. those are all in the dock. see what I mean?

sure you can disable the dock, by just closing it, close that, close the browser,close the inspector, close them all if you want.

you need to manipulate screensets to do what you want to do.
rather than using one screen for everything, use your screensets for different views.