Fix cache mixing with high-dpi and low-dpi osm providers

This cache fixes the cache mixing problem that has been introduced
with the osm high dpi tiles support.

high dpi providers fall back to low dpi ones and ultimately
to hardcoded providers (also low dpi), and can also be enabled/disabled
via plugin parameter, thus leaving the cache for a given map id
dirty for the next run.

With this patch high dpi tiles are named differently from low dpi ones.
If high-dpi providers are selected, but become not available, the
cache can also change the tileset to load at runtime

Change-Id: I229692da07c1fc61c58fb0b6fae6ec5af16e43a7
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>