Important changes:
* Refactored QtRequests to accept parameters for `GET` operations
* Infer response data type from content-type header
* Support caching to disk, a manager with this set prefers the cache
* Support multiple handlers for a single request (unused, possibly pointeless)
* Subclass `ShopImageWidget` for all widgets used in the shop
* Request a resized image instead of the original one
* Fix the search and browse functions
