support lazy loading of programs

[Yujin Kim]

When I click "Games," or whatever tab, it becomes unresponsive for several seconds while loading the entire list of programs. As AppCenter grows, this will be a serious problem. There needs to be either a support for lazy loading (using the classic "multi-page" system or whatever) or a hierarchical display, or showing that it's loading (blurring the list behind while loading will be excellent).