kivy: move "lightning" (gossip) btn inside "channels" dialog

and show toast msg when opening "channels" dialog if lightning
is not available for wallet

electrum/gui/kivy/main.kv

@@ -456,9 +456,6 @@ BoxLayout:
                 ActionOvrButton:
                     name: 'network'
                     text: _('Network')
-                ActionOvrButton:
-                    name: 'lightning'
-                    text: _('Lightning')
                 ActionOvrButton:
                     name: 'addresses_dialog'
                     text: _('Addresses')

electrum/gui/kivy/main_window.py

@@ -736,6 +736,9 @@ class ElectrumWindow(App, Logger):
                 ref.data = xpub
                 master_public_keys_layout.add_widget(ref)
             popup.open()
+        elif name == 'lightning_channels_dialog' and not self.wallet.can_have_lightning():
+            self.show_error(_("Not available for this wallet.") + "\n\n" +
+                            _("Lightning is currently restricted to HD wallets with p2wpkh addresses."))
         elif name.endswith("_dialog"):
             getattr(self, name)()
         else:

electrum/gui/kivy/uix/dialogs/lightning_channels.py

@@ -90,8 +90,13 @@ Builder.load_string(r'''
             size_hint: 1, None
             height: '48dp'
             Widget:
-                size_hint: 0.7, None
+                size_hint: 0.4, None
                 height: '48dp'
+            Button:
+                size_hint: 0.3, None
+                height: '48dp'
+                text: _('Show Gossip')
+                on_release: popup.app.popup_dialog('lightning')
             Button:
                 size_hint: 0.3, None
                 height: '48dp'