BFM demod: Delete the SpectrumVis when destroying BFMDemodGUI

Take care of a pair of memory leaks that occur when the BFM demod GUI
is closed.

==786== 19,952 (24 direct, 19,928 indirect) bytes in 1 blocks are definitely lost in loss record 7,065 of 7,081
==786==    at 0x4C2D54F: operator new(unsigned long) (vg_replace_malloc.c:334)
==786==    by 0x54C82ED: allocate (new_allocator.h:111)
==786==    by 0x54C82ED: allocate (alloc_traits.h:436)
==786==    by 0x54C82ED: _M_get_node (stl_list.h:383)
==786==    by 0x54C82ED: _M_create_node<FFTWEngine::Plan* const&> (stl_list.h:572)
==786==    by 0x54C82ED: _M_insert<FFTWEngine::Plan* const&> (stl_list.h:1801)
==786==    by 0x54C82ED: push_back (stl_list.h:1118)
==786==    by 0x54C82ED: FFTWEngine::configure(int, bool) (fftwengine.cpp:35)
==786==    by 0x4F4590B: SpectrumVis::handleConfigure(int, int, FFTWindow::Function) (spectrumvis.cpp:206)
==786==    by 0x4F459EE: SpectrumVis::handleMessage(Message const&) (spectrumvis.cpp:170)
==786==    by 0x548346C: BasebandSampleSink::handleInputMessages() (basebandsamplesink.cpp:21)
==786==    by 0x69E1615: QMetaObject::activate(QObject*, int, int, void**) (in /usr/lib/libQt5Core.so.5.10.1)
==786==    by 0x5497D4B: MessageQueue::push(Message*, bool) (messagequeue.cpp:52)
==786==    by 0x33F9D61A: BFMDemodGUI::BFMDemodGUI(PluginAPI*, DeviceUISet*, BasebandSampleSink*, QWidget*) (bfmdemodgui.cpp:352)
==786==    by 0x33F9D985: BFMDemodGUI::create(PluginAPI*, DeviceUISet*, BasebandSampleSink*) (bfmdemodgui.cpp:50)
==786==    by 0x33FB035D: non-virtual thunk to BFMPlugin::createRxChannelGUI(DeviceUISet*, BasebandSampleSink*) (bfmplugin.cpp:57)
==786==    by 0x4F47F19: DeviceUISet::loadRxChannelSettings(Preset const*, PluginAPI*) (deviceuiset.cpp:201)
==786==    by 0x4EA51EA: MainWindow::loadPresetSettings(Preset const*, int) (mainwindow.cpp:575)
==786==    by 0x4EAC81B: MainWindow::MainWindow(qtwebapp::LoggerWithFile*, MainParser const&, QWidget*) (mainwindow.cpp:176)
==786==    by 0x10A49B: runQtApplication(int, char**, qtwebapp::LoggerWithFile*) (main.cpp:120)
==786==    by 0x109B38: main (main.cpp:131)
==786==
==786== 54,096 (112 direct, 53,984 indirect) bytes in 1 blocks are definitely lost in loss record 7,075 of 7,081
==786==    at 0x4C2D54F: operator new(unsigned long) (vg_replace_malloc.c:334)
==786==    by 0x69E9162: QObject::QObject(QObject*) (in /usr/lib/libQt5Core.so.5.10.1)
==786==    by 0x5497BEE: MessageQueue::MessageQueue(QObject*) (messagequeue.cpp:26)
==786==    by 0x54834FF: BasebandSampleSink::BasebandSampleSink() (basebandsamplesink.cpp:6)
==786==    by 0x4F45A29: SpectrumVis::SpectrumVis(float, GLSpectrum*) (spectrumvis.cpp:26)
==786==    by 0x33F9D53C: BFMDemodGUI::BFMDemodGUI(PluginAPI*, DeviceUISet*, BasebandSampleSink*, QWidget*) (bfmdemodgui.cpp:342)
==786==    by 0x33F9D985: BFMDemodGUI::create(PluginAPI*, DeviceUISet*, BasebandSampleSink*) (bfmdemodgui.cpp:50)
==786==    by 0x33FB035D: non-virtual thunk to BFMPlugin::createRxChannelGUI(DeviceUISet*, BasebandSampleSink*) (bfmplugin.cpp:57)
==786==    by 0x4F47F19: DeviceUISet::loadRxChannelSettings(Preset const*, PluginAPI*) (deviceuiset.cpp:201)
==786==    by 0x4EA51EA: MainWindow::loadPresetSettings(Preset const*, int) (mainwindow.cpp:575)
==786==    by 0x4EAC81B: MainWindow::MainWindow(qtwebapp::LoggerWithFile*, MainParser const&, QWidget*) (mainwindow.cpp:176)
==786==    by 0x10A49B: runQtApplication(int, char**, qtwebapp::LoggerWithFile*) (main.cpp:120)
==786==    by 0x109B38: main (main.cpp:131)

plugins/channelrx/demodbfm/bfmdemodgui.cpp

@@ -388,6 +388,7 @@ BFMDemodGUI::~BFMDemodGUI()
 {
     m_deviceUISet->removeRxChannelInstance(this);
 	delete m_bfmDemod; // TODO: check this: when the GUI closes it has to delete the demodulator
+	delete m_spectrumVis;
 	delete ui;
 }