Each configuration file is kept in separate QSettings object and lookup
for values is done in load order - which is reverse from convention
where 10-something.conf can be overridden with 20-something-else.conf.

Change evaluation so that latter files can override earlier ones under
assumption that so far there have been no devices that would actually
employ more than one config.d file and thus glitches are unlikely.

Also merge data from all settings files into a single QSettings object
i.e. trade a bit more complex startup for simpler lookups during runtime.

The primary config file, if specified, still overrides any settings made
from config.d files.

Rename 90-sensord-default.conf to 10-sensord-default.conf so that it
still is 10 points up from the minimum priority.
Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>