Mimic Libadwaita window and headerbar style

[Jupi007]

• Remove prominent dark window border also on light theme ;
• Adjust window box-shadow ;
• Reduce window controls spacing ;
• Use Libadwaita colors and size in the headerbar.

	Focused	Backdrop
Light
Dark

The backdrop headerbar should take the window background colour (it is fine in the light theme).
@Feichtmeier, we need to define which colour use to replace the current one (related to ubuntu/yaru.dart#234), so I can also use it here.

[Jupi007]

@Feichtmeier What do you think of merge this as it awaiting that we decide something about the dark theme background.
In fact, there is some really interesting thing in this PR I would like to get in (like the removing of the dark border in light theme - like that we did previously in dark theme).

[Feichtmeier]

yes looks nice

[Jupi007]

Okay, let's merge so