wallet.get_max_amount method, used by qt and kivy

gui/kivy/main_window.py

@@ -440,6 +440,11 @@ class ElectrumWindow(App):
         status_card.uncomfirmed = unconfirmed.strip()
 
 
+    def get_max_amount(self):
+        inputs = self.wallet.get_spendable_coins(None)
+        amount, fee = self.wallet.get_max_amount(self.electrum_config, inputs, None)
+        return self.format_amount(amount)
+
     def update_amount(self, amount, c):
         if c == '<':
             return amount[:-1]

gui/kivy/uix/ui_screens/amount.kv

@@ -22,8 +22,9 @@ Popup:
                 Button:
                     size_hint_x: 1
                     height: '48dp'
-                    text: ''
-                    on_release: a.value = "max"
+                    amount: app.get_max_amount()
+                    text: self.amount + ' ' + app.base_unit
+                    on_release: a.amount = self.amount
 
             BoxLayout:
                 size_hint: 1, None