feat: Add new Shortcut for toggling the chat list

<Primary>R show/hides the chat list

gajim/data/gui/shortcuts_window.ui

@@ -175,6 +175,13 @@
           <object class="GtkShortcutsGroup">
             <property name="visible">1</property>
             <property name="title" translatable="yes">Tabs</property>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="visible">1</property>
+                <property name="accelerator">&lt;primary&gt;R</property>
+                <property name="title" translatable="yes">Toggle Chat List</property>
+              </object>
+            </child>
             <child>
               <object class="GtkShortcutsShortcut">
                 <property name="visible">1</property>

gajim/data/other/shortcuts.json

@@ -30,5 +30,6 @@
     "win.switch-tab-7": ["<Alt>7", "<Alt>KP_7"],
     "win.switch-tab-8": ["<Alt>8", "<Alt>KP_8"],
     "win.switch-tab-9": ["<Alt>9", "<Alt>KP_9"],
-    "win.search-history": ["<Primary>H"]
+    "win.search-history": ["<Primary>H"],
+    "win.hide-chat-list": ["<Primary>R"]
 }

gajim/gtk/chat_page.py

@@ -313,3 +313,8 @@ def hide_search(self) -> bool:
             self._search_revealer.set_reveal_child(False)
             return True
         return False
+
+    def toggle_chat_list(self) -> None:
+        chat_list = self._ui.paned.get_child1()
+        assert chat_list is not None
+        chat_list.set_visible(not chat_list.get_visible())

gajim/gtk/main.py

@@ -343,6 +343,7 @@ def _add_actions2(self) -> None:
             'switch-tab-7',
             'switch-tab-8',
             'switch-tab-9',
+            'toggle-chat-list',
         ]
 
         disabled_for_emacs = (
@@ -403,6 +404,9 @@ def _on_action(self,
             number = int(action_name[-1]) - 1
             self.select_chat_number(number)
 
+        elif action_name == 'toggle-chat-list':
+            self._chat_page.toggle_chat_list()
+
         return None
 
     def _on_window_motion_notify(self,