MainWindow: Separate the container background from the window

[lenemter]

Supersedes #180
Fixes #275

[leolost2605]

The gap is the reason I opened #307

[teamcons]

Would it be possible to rework this one to take #307 into account, and still separate container ?
I would really appreciate separating container from window

[leolost2605]

What do you mean by container separation? That it looks like the old dock or that it's just an additional widget?

[alainm23]

remove_css_class ("background");

[teamcons]

What do you mean by container separation? That it looks like the old dock or that it's just an additional widget?

Not sure of the Vala aspect or if i understtood correctly so this may not be super articulate, but at the time, i cannot seem to specify in a CSS override a background that is smaller/does not englobe launchers - Such as the one Plank has by default.
It is fixed and cannot draw whatever - Because it also is the main window itself
some people will want the old plank style - and allowing to draw a background which isnt the one that has to contain everything, would allow third parties to offer it, and shift any critic like "i want it like this or that or whatever" to "ask the community lol"