ManagerToolbar: Set toolbar buttons to half opaciy when not sensitive

There is otherwise no indication when there is no text show.

blueman/gui/manager/ManagerToolbar.py

@@ -78,17 +78,22 @@ def _update_buttons(self, adapter: Optional[Adapter]) -> None:
         self.b_search.props.sensitive = powered and not (adapter and adapter["Discovering"])
 
         tree_iter = self.blueman.List.selected()
+        opacity = 0.5
         if tree_iter is None:
             self.b_bond.props.sensitive = False
+            self.b_bond.props.opacity = opacity
             self.b_trust.props.sensitive = False
             self.b_remove.props.sensitive = False
             self.b_send.props.sensitive = False
+            self.b_send.props.opacity = opacity
         else:
             row = self.blueman.List.get(tree_iter, "paired", "trusted", "objpush")
             self.b_bond.props.sensitive = powered and not row["paired"]
+            self.b_bond.props.opacity = 1.0 if powered and not row["paired"] else opacity
             self.b_trust.props.sensitive = True
             self.b_remove.props.sensitive = True
             self.b_send.props.sensitive = powered and row["objpush"]
+            self.b_send.props.opacity = 1.0 if powered and row["objpush"] else opacity
 
             self.b_trust.props.icon_name = "blueman-untrust-symbolic" if row["trusted"] else "blueman-trust-symbolic"
             self.b_trust.props.label = _("Untrust") if row["trusted"] else _("Trust")