Preferences: When using the Restore to defaults button for the Tweaks, remove all plugin tweaks as well. This allows the user to recover the tweaks file if one of the plugins insert invalid code into the plugin tweaks

src/calibre/gui2/preferences/tweaks.py

@@ -202,6 +202,7 @@ class Tweaks(QAbstractListModel, SearchQueryParser):  # {{{
     def restore_to_defaults(self):
         for r in range(self.rowCount()):
             self.restore_to_default(self.index(r))
+        self.plugin_tweaks = {}
 
     def update_tweak(self, idx, varmap):
         tweak = self.data(idx, Qt.UserRole)