fix preview button shortcut

Qt 'helpfully' erases any existing shortcuts when .setText() is called,
and as PyQt's generated forms set the text after the shortcut is set,
it gets lost.

https://anki.tenderapp.com/discussions/ankidesktop/38038-does-ctrlshiftp-doesnt-trigger-preview-in-browse

qt/aqt/browser.py

@@ -597,6 +597,7 @@ class Browser(QMainWindow):
         f.previewButton.setToolTip(
             _("Preview Selected Card (%s)") % shortcut(_("Ctrl+Shift+P"))
         )
+        f.previewButton.setShortcut("Ctrl+Shift+P")
 
         f.filter.clicked.connect(self.onFilterButton)
         # edit