stack: 'return' in configure for windows not owned by this layout

The first check in the configure function tries to find the window in
the stacks, and if it's not found, it hides it. This commit adds a
return after hiding it, because there's no reason to continue
configuring it.
drawer: cleanup - turn draw_fill into draw(fill=True)

The body of both functions looked copypasted.
group: Catch and avoid propagating exceptions in layout()

Just a way to prevent misbehaving layouts from breaking more than they
should.
default_config: Stack has a 'num_stacks' param, not 'stacks'