layout: VerticalTile: add margin support

Signed-off-by: Florian Scherf <fscherf@gmx.net>
Check the results of ConfigureWindow requests

This fixes a similar issue that 71a8d0d04602c53dd05e15a3cf8dde0a00e1fc97 does.
In some cases, we have 'dead' windows that we attempt to operate on. If we just
send unchecked requests, we get the exceptions back in the poll loop and
they're rendered in the users' logs. Instead, we should check them and fail
immediately. The previous patch changes the logging scheme slightly so that
these kinds of errors are not rendered.

Closes #539
Revert "Check the results of ConfigureWindow requests"

This reverts commit ac1ba76579aea05128199e92fc2d3411c912f0f7.

As is noted in #551, this breaks several things and was committed mostly by
accident. We still want this patch, it just needs more work.
Skip test_resize and test_resize_

We have an open issue for skipping these (#509), and peoples' PRs are failing
(and the build is failing) because of them. We should just ignore these tests
until someone actually fixes them.
Move handing of negative numbers in ConfigureNotify

Change how negative numbers are handled for ConfigureNotify such that we are
still calling the xcffib version of ConfigureNotify, but with bit-twiddled
values that will pack negative and positive numbers correctly.
Add pypy3 tests to Travis

Closes #552
Change default wmname to LG3D

Based on #558 (and lots of other issues in the past, including previous
complaints about pycharm not working out of the box), we should just default to
something that actually works here. This should have essentially no effect for
anyone but users of java apps, who will experience them working correctly :)
Added Net widget that displays Up and Down speed.

Closes #559
Squashed cAllow spaces in ThermalSensors sensor tag

Closes #556
Closes #554
manager: expose the executor
widget:  create an executor based text box

Text box that can wrap calling blocking functions which on return will
provide updated text.  This uses the default eventloop executor to wrap
the functions within a coroutine.
mpdwidget:  use ThreadPoolText

Rather that updating at some set interval, use the mpd idle function to
wait for new events to occur and then update the widget text.  Use the
ThreadPoolText class in order to wrap the idle wait inside of a
coroutine.

Also other minor pep8 related cleanup.

Closes #565
A draft of a potential cffi-dbus fix

This is (as described in the many comments) a massive hack, but it might Just
Work.
wip