MainWindow: call cleanup code on aboutToQuit signal

Address the two largest leaks detected by valgrind when simply opening
and closing sdrangel with the 'X' button rather than the 'Exit' menu
item.

==11533== 18,016 (24 direct, 17,992 indirect) bytes in 1 blocks are definitely lost in loss record 6,991 of 7,005
==11533==    at 0x4C2D54F: operator new(unsigned long) (vg_replace_malloc.c:334)
==11533==    by 0x54C82ED: allocate (new_allocator.h:111)
==11533==    by 0x54C82ED: allocate (alloc_traits.h:436)
==11533==    by 0x54C82ED: _M_get_node (stl_list.h:383)
==11533==    by 0x54C82ED: _M_create_node<FFTWEngine::Plan* const&> (stl_list.h:572)
==11533==    by 0x54C82ED: _M_insert<FFTWEngine::Plan* const&> (stl_list.h:1801)
==11533==    by 0x54C82ED: push_back (stl_list.h:1118)
==11533==    by 0x54C82ED: FFTWEngine::configure(int, bool) (fftwengine.cpp:35)
==11533==    by 0x4F4590B: SpectrumVis::handleConfigure(int, int, FFTWindow::Function) (spectrumvis.cpp:206)
==11533==    by 0x4F45B94: SpectrumVis::SpectrumVis(float, GLSpectrum*) (spectrumvis.cpp:29)
==11533==    by 0x4F46349: DeviceUISet::DeviceUISet(int, bool, QTimer&) (deviceuiset.cpp:39)
==11533==    by 0x4EA890E: MainWindow::addSourceDevice(int) (mainwindow.cpp:240)
==11533==    by 0x4EAC7F3: MainWindow::MainWindow(qtwebapp::LoggerWithFile*, MainParser const&, QWidget*) (mainwindow.cpp:171)
==11533==    by 0x10A3B3: runQtApplication(int, char**, qtwebapp::LoggerWithFile*) (main.cpp:120)
==11533==    by 0x109A78: main (main.cpp:130)
==11533==
==11533== 54,096 (112 direct, 53,984 indirect) bytes in 1 blocks are definitely lost in loss record 6,999 of 7,005
==11533==    at 0x4C2D54F: operator new(unsigned long) (vg_replace_malloc.c:334)
==11533==    by 0x69E9162: QObject::QObject(QObject*) (in /usr/lib/libQt5Core.so.5.10.1)
==11533==    by 0x54834E6: BasebandSampleSink::BasebandSampleSink() (basebandsamplesink.cpp:6)
==11533==    by 0x4F45A29: SpectrumVis::SpectrumVis(float, GLSpectrum*) (spectrumvis.cpp:26)
==11533==    by 0x4F46349: DeviceUISet::DeviceUISet(int, bool, QTimer&) (deviceuiset.cpp:39)
==11533==    by 0x4EA890E: MainWindow::addSourceDevice(int) (mainwindow.cpp:240)
==11533==    by 0x4EAC7F3: MainWindow::MainWindow(qtwebapp::LoggerWithFile*, MainParser const&, QWidget*) (mainwindow.cpp:171)
==11533==    by 0x10A3B3: runQtApplication(int, char**, qtwebapp::LoggerWithFile*) (main.cpp:120)
==11533==    by 0x109A78: main (main.cpp:130)

app/main.cpp

@@ -119,6 +119,7 @@ static int runQtApplication(int argc, char* argv[], qtwebapp::LoggerWithFile *lo
 
 	MainWindow w(logger, parser);
 	w.show();
+	QObject::connect(&a, SIGNAL(aboutToQuit()), &w, SLOT(on_action_Exit_triggered()));
 
 	return a.exec();
 }