kivy: use on_release instead of on_press in buttons

I think on_release has better UX.
More importantly, on desktop linux with kivy 2.0.0rc2, on_press does not work for me.
(but on Android, with kivy 1.11.1, it does)

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

@@ -36,7 +36,7 @@ Builder.load_string('''
             text: 'Show report contents'
             height: '48dp'
             size_hint: 1, None
-            on_press: root.show_contents()
+            on_release: root.show_contents()
         BoxLayout:
             size_hint: 1, 0.1
         Label:

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

@@ -94,7 +94,7 @@ Builder.load_string(r'''
                 size_hint: 0.3, None
                 height: '48dp'
                 text: _('New...')
-                on_press: popup.app.popup_dialog('lightning_open_channel_dialog')
+                on_release: popup.app.popup_dialog('lightning_open_channel_dialog')
 
 
 <ChannelDetailsPopup@Popup>: