project: fix build against latest QextSerialPort

They've improved a lot in latest QextSerialPort from master branch.
The build became much simpler and its license has been clarified to
be the MIT license. So we can use it without any doubt.

We use QextSerialPort from git commit:
3be3fbf31edf6c91c7838f585b77acd787811296
Mon Jul 8 16:33:41 2013 -0400
report: add debug output for system() call

We don't care if system() fails for opening the CSV file. It is
just an user convenience function. But as a compiler warning is
issued, put the return value into a debug message.
mainwindow: about: add author's e-mail address

It's always good style to show who to contact regarding the
software. So add the author's e-mail address to the about dialog.
de_DE: update German translation
mainwindow: add trigger rate combo box

We need a GUI element to control the trigger rate (SCPI command
"RATE"). The Tektronix DMM 4050 supports "S", "M" and "F".
de_DE: update German translation

With the new GUI element "Trigger Rate" we need to update the
translation again.
settings: optimize the use of settings

We only use class Settings as QObject for localization. It doesn't
make sense that there are multiple Settings objects created. So let
only MainWindow create it and provide a pointer to it to DMMControl
as well as SerialPortCtr. There are also too many get functions. So
use the settings protected with these three classes as friends and
access settings directly.
import DMM_send_values source

It is just a test tool which simulates a Tektronix 4050 DMM. This
is for developers only and is useful if there is only limited
access to such a DMM. All you need is a null modem cable and two
serial ports.