Implement basic functionality of tiled App Catalog.

Implement both Index page and App Details page. Right now it uses fake
model application, but UI render part is complete.  There is no error
handling here yet, it should be added later.

Also basic image handling (for icons) is implemented:
* added images for app entries and details page.
* added image caching logic to store images on local disk.
* added Content-Cache header to cache images on client side too.

Partially implements: blueprint app-catalog-ui
Change-Id: Ibaff5b7805b6fa45906e6247619483af924889d9
Merge "Implement basic functionality of tiled App Catalog."