Integrate eth_accounts and eth_transact to use new account manager

* Add from to eth_transact / xeth.Transact and add static pass in lieu
  of integrating with native Mist window for user passphrase entry
* Make eth_accounts return AccountManager.Accounts()
* Add a Generate Key menu item in Mist

cmd/mist/assets/qml/main.qml

@@ -190,6 +190,11 @@ ApplicationWindow {
                 }
             }
 
+            MenuItem {
+                text: "Generate key"
+                shortcut: "Ctrl+k"
+                onTriggered: gui.generateKey()
+            }
         }
 
         Menu {