plotter: clear waterfall using Delete key

The waterfall is not automatically cleared after changes to the many
settings that could cause it to be distorted or invalid. An explicit
clear allows users who do not want to see outdated data to get rid of
it.

Signed-off-by: Jeff Long <willcode4@gmail.com>

resources/kbd-shortcuts.txt

@@ -12,6 +12,7 @@ Main window actions:
  F11            Toggle full screen mode
  F              Set focus to the frequency controller
  Z              Zero frequency offset
+ Delete         Clear waterfall
  Ctrl+Q         Quit the program
 
 Receiver modes:

src/applications/gqrx/mainwindow.cpp

@@ -172,6 +172,9 @@ MainWindow::MainWindow(const QString& cfgfile, bool edit_conf, QWidget *parent)
     // toggle markers on/off
     auto *toggle_markers_shortcut = new QShortcut(QKeySequence(Qt::CTRL | Qt::Key_K), this);
     QObject::connect(toggle_markers_shortcut, &QShortcut::activated, this, &MainWindow::toggleMarkers);
+    // clear waterfall
+    auto *clear_waterfall_shortcut = new QShortcut(Qt::Key_Delete, this);
+    QObject::connect(clear_waterfall_shortcut, SIGNAL(activated()), ui->plotter, SLOT(clearWaterfall()));
 
     setCorner(Qt::TopLeftCorner, Qt::LeftDockWidgetArea);
     setCorner(Qt::TopRightCorner, Qt::RightDockWidgetArea);

src/qtgui/plotter.cpp

@@ -2455,6 +2455,13 @@ void CPlotter::setMarkers(qint64 a, qint64 b)
     updateOverlay();
 }
 
+void CPlotter::clearWaterfall()
+{
+    if (!m_WaterfallPixmap.isNull()) {
+        m_WaterfallPixmap.fill(Qt::black);
+    }
+}
+
 void CPlotter::calcDivSize (qint64 low, qint64 high, int divswanted, qint64 &adjlow, qint64 &step, int& divs)
 {
     qCDebug(plotter) << "low:" << low;

src/qtgui/plotter.h

@@ -181,6 +181,7 @@ public slots:
     void enableBandPlan(bool enable);
     void enableMarkers(bool enabled);
     void setMarkers(qint64 a, qint64 b);
+    void clearWaterfall();
     void updateOverlay();
 
     void setPercent2DScreen(int percent)