Fix #443

Looks like these functions were removed by
d87967ed4001d8f222c24827b43b31240795be4c and not tested so nothing caught it
:-(
clock widget supports other timezones
Add background gradient documentation
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'
add support to display the volume(widget) states as emoji characters
Don't import ThermalSensor twice
Drop Xlib from tests, use xcb

Closes #447
Enable travis tests
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