pref: general: connect comboBox_UnitSystem manually. Fix System unit tooltip string

src/Gui/DlgGeneral.ui

@@ -80,7 +80,7 @@
          <item row="1" column="1">
           <widget class="QComboBox" name="comboBox_UnitSystem">
            <property name="toolTip">
-            <string>Unit system that should be used for all parts the application</string>
+            <string>Unit system that should be used for all parts of the application</string>
            </property>
           </widget>
          </item>

src/Gui/DlgGeneralImp.cpp

@@ -83,6 +83,7 @@ DlgGeneralImp::DlgGeneralImp( QWidget* parent )
     ui->RevertToSavedConfig->setEnabled(backups.empty());
     connect(ui->RevertToSavedConfig, &QPushButton::clicked, this, &DlgGeneralImp::revertToSavedConfig);
 
+    connect(ui->comboBox_UnitSystem, qOverload<int>(&QComboBox::currentIndexChanged), this, &DlgGeneralImp::onUnitSystemIndexChanged);
     ui->spinBoxDecimals->setMaximum(std::numeric_limits<double>::digits10 + 1);
 
     int num = static_cast<int>(Base::UnitSystem::NumUnitSystemTypes);
@@ -536,8 +537,7 @@ void DlgGeneralImp::onLoadPreferencePackClicked(const std::string& packName)
     }
 }
 
-
-void DlgGeneralImp::on_comboBox_UnitSystem_currentIndexChanged(int index)
+void DlgGeneralImp::onUnitSystemIndexChanged(int index)
 {
     if (index < 0)
         return; // happens when clearing the combo box in retranslateUi()

src/Gui/DlgGeneralImp.h

@@ -65,7 +65,7 @@ protected Q_SLOTS:
     void onImportConfigClicked();
 
 public Q_SLOTS:
-    void on_comboBox_UnitSystem_currentIndexChanged(int index);
+    void onUnitSystemIndexChanged(int index);
 
 private:
     void setRecentFileSize();