dbhub: Add proxy configuration support

This adds a new dialog, accessible from the Remote tab in the Preferences
Dialog, which allows the user to configure the proxy to use for all
network connections.

The new default is to use the system-wide proxy settings.

See issue #979.

src/PreferencesDialog.h

@@ -8,6 +8,8 @@ class QFrame;
 class QSslCertificate;
 class QAbstractButton;
 
+class ProxyDialog;
+
 namespace Ui {
 class PreferencesDialog;
 }
@@ -44,12 +46,15 @@ private slots:
     void chooseRemoteCloneDirectory();
     void updatePreviewFont();
     void adjustColorsToStyle(int style);
+    void configureProxy();
 
     void on_buttonManageFileExtension_clicked();
     void on_buttonBox_clicked(QAbstractButton* button);
 
 private:
-    Ui::PreferencesDialog *ui;
+    Ui::PreferencesDialog* ui;
+
+    ProxyDialog* m_proxyDialog;
 
     QStringList m_dbFileExtensions;