Dark/Light styles: upgraded to QDarkStyle v3.2.1

This fixes at least a problem in display of combo-boxes.

Updated following instructions in:
https://github.com/ColinDuquesnoy/QDarkStyleSheet/tree/v3.2.1#c

- Related issue: #2558.
- Issue still present (no difference in toggled buttons in menus): #1899.

src/MainWindow.cpp

@@ -2349,7 +2349,7 @@ void MainWindow::reloadSettings()
         break;
     case Settings::DarkStyle :
     case Settings::LightStyle :
-        QFile f(style == Settings::DarkStyle ? ":qdarkstyle/dark/style.qss" : ":qdarkstyle/light/style.qss");
+        QFile f(style == Settings::DarkStyle ? ":qdarkstyle/dark/darkstyle.qss" : ":qdarkstyle/light/lightstyle.qss");
         if (!f.exists()) {
             QMessageBox::warning(this, qApp->applicationName(),
                                tr("Could not find resource file: %1").arg(f.fileName()));

src/qdarkstyle/dark/darkstyle.qrc

@@ -1,7 +1,6 @@
 
 <RCC warning="WARNING! File created programmatically. All changes made in this file will be lost!">
   <qresource prefix="qss_icons/dark">
-    <file>rc/.keep</file>
     <file>rc/arrow_down.png</file>
     <file>rc/arrow_down@2x.png</file>
     <file>rc/arrow_down_disabled.png</file>
@@ -212,6 +211,6 @@
     <file>rc/window_undock_pressed@2x.png</file>
   </qresource>
   <qresource prefix="qdarkstyle/dark">
-      <file>style.qss</file>
+      <file>darkstyle.qss</file>
   </qresource>
 </RCC>