add keyboard shortcut for plot filtering

src/ui/Linechart.ui

@@ -221,7 +221,7 @@
        <item row="1" column="0">
         <widget class="QLineEdit" name="plotFilterLineEdit">
          <property name="placeholderText">
-          <string>Filter...</string>
+          <string>Filter... (Ctrl+F)</string>
          </property>
         </widget>
        </item>

src/ui/linechart/LinechartWidget.cc

@@ -106,6 +106,7 @@ LinechartWidget::LinechartWidget(int systemid, QWidget *parent) : QWidget(parent
     connect(ui.recolorButton, SIGNAL(clicked()), this, SLOT(recolor()));
     connect(ui.shortNameCheckBox, SIGNAL(clicked(bool)), this, SLOT(setShortNames(bool)));
     connect(ui.plotFilterLineEdit, SIGNAL(textChanged(const QString&)), this, SLOT(filterCurves(const QString&)));
+    new QShortcut(QKeySequence(Qt::CTRL + Qt::Key_F), this, SLOT(setPlotFilterLineEditFocus()));
 
     int labelRow = curvesWidgetLayout->rowCount();
 
@@ -694,6 +695,11 @@ void LinechartWidget::recolor()
     }
 }
 
+void LinechartWidget::setPlotFilterLineEditFocus()
+{
+    ui.plotFilterLineEdit->setFocus(Qt::ShortcutFocusReason);
+}
+
 void LinechartWidget::filterCurve(const QString &key, bool match)
 {
         colorIcons[key]->setVisible(match);

src/ui/linechart/LinechartWidget.h

@@ -108,6 +108,8 @@ public slots:
     void readSettings();
     /** @brief Select all curves */
     void selectAllCurves(bool all);
+    /** @brief Sets the focus to the LineEdit for plot-filtering */
+    void setPlotFilterLineEditFocus();
 
 private slots:
     /** Called when the user changes the time scale combobox. */