config: abstract away mempool depth format str "%.1f MB from tip"

related: 4dd94172a6
(as that commit introduced depth targets that need more precision to display)

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

@@ -125,6 +125,7 @@ class TxDialog(Factory.Popup):
         self.app = app  # type: ElectrumWindow
         self.wallet = self.app.wallet
         self.tx = tx  # type: Transaction
+        self.config = self.app.electrum_config
 
         # If the wallet can populate the inputs with more info, do it now.
         # As a result, e.g. we might learn an imported address tx is segwit,
@@ -154,7 +155,7 @@ class TxDialog(Factory.Popup):
             self.date_str = datetime.fromtimestamp(tx_mined_status.timestamp).isoformat(' ')[:-3]
         elif exp_n is not None:
             self.date_label = _('Mempool depth')
-            self.date_str = _('{} from tip').format('%.2f MB'%(exp_n/1000000))
+            self.date_str = self.config.depth_tooltip(exp_n)
         else:
             self.date_label = ''
             self.date_str = ''