1 - you don't need to regist panel if you want drag it
you will be able drag panel when dragPanelBtn() or dragPanelBgr() will by asignet to it's button_click event or on_click event

function panelRegist() is used when you wish to make some panel to work as "catcher" screen border works allways as "catcher" (at least in this code version)
I will try explane it like this:

Let's take demo panel r
Try to drag red panel. You should see that it stick to sceen border and to green panels border but it never sticks to blue panels border. Why is that so?
It's becouse blue panel is not registed as "catcher" and during drag action
blue panels position is not calculated

Look at demo main function I think you will understand
(maybe terminology "catcher" is bad but I don't know how to name it)

2 - it's realy good idea