main: Use HdyApplicationWindow instead of GtkApplicationWindow

So here is the transfer to HdyApplicationWindow.

Please note that after this MR there is more to do.
What I intend to do is to merge the main header with the fullscreen header.
They're totally the same except for the position of the fullscreen.
For that I will use the HdyFlap just like epiphany does.

If you do not agree, please let met know.

Thanks !