Add new global shortcut system backend for X11 and Windows

- Remove qxt
- Also create an option for enabled/disabling shortcuts through X11.

src/widgets/stretchheaderview.cpp

@@ -246,11 +246,8 @@ void StretchHeaderView::SetColumnWidth(int logical, ColumnWidthType width) {
 bool StretchHeaderView::RestoreState(const QByteArray& data) {
 
   QDataStream s(data);
-#if QT_VERSION < QT_VERSION_CHECK(5, 6, 0)
-  s.setVersion(QDataStream::Qt_5_5);
-#else
+
   s.setVersion(QDataStream::Qt_5_6);
-#endif
 
   int magic_number = 0;
   s >> magic_number;
@@ -314,11 +311,8 @@ QByteArray StretchHeaderView::SaveState() const {
     visual_indices << logicalIndex(i);
   }
 
-#if QT_VERSION < QT_VERSION_CHECK(5, 6, 0)
-  s.setVersion(QDataStream::Qt_5_5);
-#else
   s.setVersion(QDataStream::Qt_5_6);
-#endif
+
   s << kMagicNumber;
 
   s << stretch_enabled_;