[FIX] sap.ui.unified.Menu: Improve ARIA support

Fixes https://github.com/SAP/openui5/issues/237

Change-Id: I2b0d8e5c3650c94f8a4e5740a970061f55782f92
CR-Id: 012006153200015523382014
Merge "[FIX] sap.ui.unified.Menu: Improve ARIA support" into rel-1.26