add 'untagged', and make clicking on 'tags' show all tagged cards

ftl/core/browsing.ftl

@@ -128,3 +128,4 @@ browsing-added-today = Added
 browsing-again-today = Again
 browsing-edited-today = Edited
 browsing-sidebar-due-today = Due
+browsing-sidebar-untagged = Untagged

qt/aqt/sidebar.py

@@ -46,6 +46,7 @@ class SidebarItemType(Enum):
     NOTETYPE = auto()
     NOTETYPE_TEMPLATE = auto()
     TAG_ROOT = auto()
+    TAG_NONE = auto()
     TAG = auto()
 
     CUSTOM = auto()
@@ -785,6 +786,16 @@ class SidebarTreeView(QTreeView):
             collapse_key=Config.Bool.COLLAPSE_TAGS,
             type=SidebarItemType.TAG_ROOT,
         )
+        root.on_click = self._filter_func(
+            self.col.build_search_string(SearchTerm(tag="none"), negate=True)
+        )
+        root.add_simple(
+            name=tr(TR.BROWSING_SIDEBAR_UNTAGGED),
+            icon=icon,
+            type=SidebarItemType.TAG_NONE,
+            on_click=self._filter_func(SearchTerm(tag="none")),
+        )
+
         render(root, tree.children)
 
     # Tree: Decks
@@ -881,6 +892,8 @@ class SidebarTreeView(QTreeView):
             return
         self.show_context_menu(item, idx)
 
+    # idx is only None when triggering the context menu from a left click on
+    # saved searches - perhaps there is a better way to handle that?
     def show_context_menu(self, item: SidebarItem, idx: Optional[QModelIndex]) -> None:
         m = QMenu()