Properly handle include cycles

Properly handle include cycles

Currently lupdate is just issuing a warning when it detects an
include cycle. Since it's not a full preprocessor, most of those
warnings are false positives, but they need special handling, this
patch provides that.

Now every result is stored as part of a cycle (so in the general
case a cycle of one file). When detecting a cycle, instead instead
of issuing a warning, we record the presence of the newly detected
cycle for later use and merge the other relevant cycles with it.
When retrieving results now it's not for a single file anymore but
for a complete cycle at once (which can turn out to be a cycle of
one file).

Task-number: QTBUG-6587

Change-Id: Ia12bc680e645b9704a3178e8a843a554a60811bf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>