Fix termination of tor process on Windows

We can't cleanly tell the process to exit on windows, but it's owned by
the control connection, so it will exit by itself. QProcess will warn,
but it's not an issue for Windows.
Fix font sizes for Windows

Avoid hardcoding point sizes in many places, since they're unreliable
across platforms. Also use a different default point size for some text
on Windows, where the normal UI default is too small.