Expose new import/export code via prefs checkbox

qt/aqt/forms/preferences.ui

@@ -7,7 +7,7 @@
     <x>0</x>
     <y>0</y>
     <width>640</width>
-    <height>518</height>
+    <height>530</height>
    </rect>
   </property>
   <property name="windowTitle">
@@ -96,6 +96,13 @@
          </property>
         </widget>
        </item>
+       <item>
+        <widget class="QCheckBox" name="new_import_export">
+         <property name="text">
+          <string notr="true">New import/export handling (beta)</string>
+         </property>
+        </widget>
+       </item>
        <item>
         <widget class="QComboBox" name="useCurrent">
          <item>
@@ -677,6 +684,7 @@
   <tabstop>pastePNG</tabstop>
   <tabstop>paste_strips_formatting</tabstop>
   <tabstop>ignore_accents_in_search</tabstop>
+  <tabstop>new_import_export</tabstop>
   <tabstop>useCurrent</tabstop>
   <tabstop>default_search_text</tabstop>
   <tabstop>uiScale</tabstop>

qt/aqt/preferences.py

@@ -219,6 +219,7 @@ class Preferences(QDialog):
         self.form.theme.addItems(themes)
         self.form.theme.setCurrentIndex(self.mw.pm.theme().value)
         qconnect(self.form.theme.currentIndexChanged, self.on_theme_changed)
+        self.form.new_import_export.setChecked(self.mw.pm.new_import_export())
 
         self.setup_language()
         self.setup_video_driver()
@@ -235,6 +236,8 @@ class Preferences(QDialog):
             self.mw.pm.setUiScale(newScale)
             restart_required = True
 
+        self.mw.pm.set_new_import_export(self.form.new_import_export.isChecked())
+
         if restart_required:
             showInfo(tr.preferences_changes_will_take_effect_when_you())