Add a button to open the account list.

Also fix the account list dialog's close button.

gui/MainWindow.ui

@@ -70,6 +70,7 @@
    <addaction name="separator"/>
    <addaction name="actionCheckUpdate"/>
    <addaction name="actionSettings"/>
+   <addaction name="actionManageAccounts"/>
    <addaction name="separator"/>
    <addaction name="actionReportBug"/>
    <addaction name="actionNews"/>
@@ -465,6 +466,14 @@
     <string>Add a new instance.</string>
    </property>
   </action>
+  <action name="actionManageAccounts">
+   <property name="text">
+    <string>Manage Accounts</string>
+   </property>
+   <property name="toolTip">
+    <string>Manage your Mojang or Minecraft accounts.</string>
+   </property>
+  </action>
  </widget>
  <layoutdefault spacing="6" margin="11"/>
  <resources>