Cache to QStandardPaths::CacheLocation

We should save our cache in the configured CacheLocation, and ensure
we are prepared for multiple BrowserContext by caching to a Default
subdir. This matches how chromium caches on linux.

Change-Id: Ibe914fca35435ea1658565be87747cfabc71e97b
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>