Add shortcuts to reverse and re-apply current sort

Fixes #1889298 [Enhancement Request: Hotkey to resort current view/reverse current sort](https://bugs.launchpad.net/calibre/+bug/1889298)

src/calibre/gui2/actions/sort.py

@@ -38,6 +38,33 @@ class SortByAction(InterfaceAction):
         self.sorted_icon = QIcon(I('ok.png'))
         self.qaction.menu().aboutToShow.connect(self.about_to_show)
 
+        def c(attr, title, tooltip, callback, keys=()):
+            ac = self.create_action(spec=(title, None, tooltip, keys), attr=attr)
+            ac.triggered.connect(callback)
+            self.gui.addAction(ac)
+            return ac
+
+        c('reverse_sort_action', _('Reverse current sort'), _('Reverse the current sort order'), self.reverse_sort)
+        c('reapply_sort_action', _('Re-apply current sort'), _('Re-apply the current sort'), self.reapply_sort, 'f5')
+
+    def reverse_sort(self):
+        lv = self.gui.library_view
+        m = lv.model()
+        try:
+            sort_col, order = m.sorted_on
+        except TypeError:
+            sort_col, order = 'date', True
+        lv.sort_by_named_field(sort_col, not order)
+
+    def reapply_sort(self):
+        lv = self.gui.library_view
+        m = lv.model()
+        try:
+            sort_col, order = m.sorted_on
+        except TypeError:
+            sort_col, order = 'date', True
+        lv.sort_by_named_field(sort_col, order)
+
     def location_selected(self, loc):
         enabled = loc == 'library'
         self.qaction.setEnabled(enabled)