Creates a viewport and registers it.
- viewportId: HTML element id of the viewport.
- currentPageId: Optional. Home page Id of the viewport. If it's not specified the first page is used.
Note: It's use is necessary only if custome viewports are used. The standar mui.viewport and mui.screen are created and registered by MobileUI that's why their are used directly.
For more details about viewports see MobileUI App Structure in User Guide.