main: use GtkBin as base instead HdyHeaderBar

So, these days I had less time than usual so I couldn't do what I had in mind like discussed in #553.

Honestly, if you allow me, I don't like very much the current solution. It bothers me a little.
But I didn't find another solution so if you have some suggestion... 👍

(last attempt) The current UI is roughly like :

• Flap
  • Toolbar (player + grilo)
  • Stack (player + grilo)

What I wanted to do :

• Stack
  • Flap
    • Player toolbar
    • Player widget
  • Grilo (grilo is a GtkBox)
    • Grilo toolbar
    • Grilo widget

Each views handle their own toolbar.
But I always have difficult to anticipate so I don't know if what i had planned was going to do more good than harm.

Thanks