Support for theming qTox tray and window icons

This patch adds ability to override qTox window and tray icons with ones from user's desktop theme. Some people prefer keeping tray icons consistent to achieve best looks.

Following theme icons used:
* qtox: general window icon which is also used in taskbar
* qtox-online, qtox-offline, qtox-busy, qtox-away, qtox-invisible, qtox-event: tray icons

If theme icon is not available then default built-in icon will be used.

src/widget/systemtrayicon.cpp

@@ -331,7 +331,7 @@ void SystemTrayIcon::setVisible(bool newState)
     }
 }
 
-void SystemTrayIcon::setIcon(QIcon &&icon)
+void SystemTrayIcon::setIcon(QIcon &icon)
 {
     if (false);
     #ifdef ENABLE_SYSTRAY_STATUSNOTIFIER_BACKEND

src/widget/systemtrayicon.h

@@ -17,7 +17,7 @@ public:
     void show();
     void hide();
     void setVisible(bool);
-    void setIcon(QIcon&& icon);
+    void setIcon(QIcon &icon);
 
 signals:
     void activated(QSystemTrayIcon::ActivationReason);

src/widget/widget.cpp

@@ -92,6 +92,10 @@ void Widget::init()
 {
     ui->setupUi(this);
 
+    QIcon themeIcon = QIcon::fromTheme("qtox");
+    if (!themeIcon.isNull())
+        setWindowIcon(themeIcon);
+
     timer = new QTimer();
     timer->start(1000);
     offlineMsgTimer = new QTimer();
@@ -224,6 +228,9 @@ void Widget::setTranslation()
 
 void Widget::updateTrayIcon()
 {
+    if (!icon)
+        return;
+
     QString status;
     if (eventIcon)
         status = "event";
@@ -233,10 +240,15 @@ void Widget::updateTrayIcon()
         if (!status.length())
             status = "offline";
     }
+
+    QIcon ico = QIcon::fromTheme("qtox-" + status);
+    if (ico.isNull())
+    {
         QString color = Settings::getInstance().getLightTrayIcon() ? "light" : "dark";
-    QString pic = ":img/taskbar/" + color + "/taskbar_" + status + ".svg";
-    if (icon)
-        icon->setIcon(QIcon(pic));
+        ico = QIcon(":img/taskbar/" + color + "/taskbar_" + status + ".svg");
+    }
+
+    icon->setIcon(ico);
 }
 
 Widget::~Widget()