kvui: always display all tab headers (#1399)

data/client.kv

@@ -1,5 +1,5 @@
 <TabbedPanel>
-    tab_width: 200
+    tab_width: root.width / app.tab_count
 <SelectableLabel>:
     canvas.before:
         Color:

kvui.py

@@ -330,6 +330,12 @@ class GameManager(App):
 
         super(GameManager, self).__init__()
 
+    @property
+    def tab_count(self):
+        if hasattr(self, "tabs"):
+            return max(1, len(self.tabs.tab_list))
+        return 1
+
     def build(self) -> Layout:
         self.container = ContainerLayout()