make update all toggle clearer

https://forums.ankiweb.net/t/anki-2-1-45-alpha/10061/59

+ remove background color; it did not look good in dark mode

ftl/qt/addons.ftl

@@ -65,3 +65,4 @@ addons-delete-the-numd-selected-addon =
        *[other] Delete the { $count } selected add-ons?
     }
 addons-choose-update-window-title = Update Add-ons
+addons-choose-update-update-all = Update All

qt/aqt/addons.py

@@ -1160,9 +1160,8 @@ class ChooseAddonsToUpdateList(QListWidget):
         qconnect(self.customContextMenuRequested, self.on_context_menu)
 
     def setup(self) -> None:
-        header_item = QListWidgetItem("", self)
+        header_item = QListWidgetItem(tr.addons_choose_update_update_all(), self)
         header_item.setFlags(Qt.ItemFlag(Qt.ItemIsUserCheckable | Qt.ItemIsEnabled))
-        header_item.setBackground(Qt.lightGray)
         self.header_item = header_item
         for update_info in self.updated_addons:
             addon_id = update_info.id