Do not dynamic cast or de-reference QObject, since object is either in
destructor or may have already been destroyed.

Using QFileDialog directly on OS X can hang intermittently due to Qt
bug.

Left QGCRadioChannelDisplay alone so that I didn’t need to modify all
the other APM board code which uses this control. Also new control only
support horizontal orientation, with no plans to add vertical.

This way we can test with QT_FAIL_WARNINGS set.

Plus a few API tweaks

I was unsure how to update the QwtPlotPrintFilter code, so I just commented it out with a FIXME.

qreal is a double on some platforms (x86) and a float on others (e.g. ARM), cast it to fix build errors like these:

src/ui/designer/QGCXYPlot.cc: In member function 'int
XYPlotCurve::appendData(const QPointF&)':
src/ui/designer/QGCXYPlot.cc:41:39: error: no matching function for call
to 'qMin(double&, qreal)'
             xmin = qMin(xmin, data.x());
                                       ^
src/ui/designer/QGCXYPlot.cc:41:39: note: candidate is:
In file included from
/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/sysroots/sama5d3_xplained/usr/include/qt4/QtCore/qnamespace.h:45:0,
                 from
/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/sysroots/sama5d3_xplained/usr/include/qt4/QtCore/qobjectdefs.h:45,
                 from
/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/sysroots/sama5d3_xplained/usr/include/qt4/QtGui/qwindowdefs.h:45,
                 from
/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/sysroots/sama5d3_xplained/usr/include/qt4/QtGui/qwidget.h:46,
                 from
/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/sysroots/sama5d3_xplained/usr/include/qt4/QtGui/qdockwidget.h:45,
                 from
/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/sysroots/sama5d3_xplained/usr/include/qt4/QtGui/QDockWidget:1,
                 from src/ui/designer/QGCXYPlot.cc:1:
/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/sysroots/sama5d3_xplained/usr/include/qt4/QtCore/qglobal.h:1213:34:
note: template<class T> const T& qMin(const T&, const T&)
 Q_DECL_CONSTEXPR inline const T &qMin(const T &a, const T &b) { return
(a < b) ? a : b; }
                                  ^

See http://www.qtcentre.org/archive/index.php/t-45475.html for some
background.

This fixes #625
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>

showLabel was connected to signal but not set as a slot

Removed for compatability

This allows you to add XY graph plots of any data.  For example,
x,y position data of a servo.