kivy wallet info screen: handle "show seed" for watch-only/no-seed case

For a watch-only wallet, previously a superfluous dummy "Tap to show"
box was shown.
For a has no seed stored (but not watch-only) wallet (e.g. bip39/xprv/imported_privkeys),
the "show seed" option was there and it raised an uncaught exception.

electrum/gui/kivy/main.kv

@@ -148,6 +148,9 @@
     font_size: '6pt'
     name: ''
     data: ''
+    visible: True
+    opacity: 1 if self.visible else 0
+    disabled: not self.visible
     text: self.data if self.data else _('Tap to show')
     touched: False
     padding: '10dp', '10dp'

electrum/gui/kivy/uix/ui_screens/status.kv

@@ -63,12 +63,13 @@ Popup:
                         data: app.wallet.get_master_public_key() or 'None'
                         name: _('Master Public Key')
                     TopLabel:
-                        text: _('This wallet is watching-only') if root.watching_only else _('Seed')
+                        text: _('This wallet is watching-only') if root.watching_only else (_('Seed') if root.has_seed else _('No seed stored for wallet'))
                     RefLabel:
                         id: seed_label
+                        visible: root.has_seed
                         data: ''
                         name: _('Seed')
-                        on_touched: if not self.data and not root.watching_only: app.show_seed(seed_label)
+                        on_touched: if not self.data and root.has_seed: app.show_seed(seed_label)
 
         BoxLayout:
             size_hint: 1, None