Allow passing unicode strings to mosquitto

_send_publish already supports encoding unicode data, so it's just a
matter of checking the types here correctly.
Pre-emptively declare some globals in windows.py

The current implementation relies on a call to init() to *declare( the global
variables.  Pyflakes doesn't / can't know this, so it assumes the usage of these
variables in the other functions in this module is erroneous.

To alleviate the warnings, declare these variables as `None` in the
module scope (where they end up anyway)
Explicitly call toInt() on this QVariant

I'm not sure why this isn't working for me out of the box.  Maybe the
type conversions must be precise?

In my case, `setSize()` expects either `(QSize)` or `(int, int)`.  `height`
is a `QVariant` containing a `float`, and this is causing my pyqt to
rais an unhandled exception.  Explicitly call toInt() to get qt to use
the proper signature when invoking setSize.