This is the first patch of a series. The final goal is to introduce
shared settings into Creator.

This particular patch shouldn't affect functionality, but it changes
a bit the design of the settings acessor and prepares the code for
the upcoming commits.

In addition it also makes restoring the settings slightly more
robust, since it will try to load a compatible "old" file from the
quick check before it actually performs the more expensive check.

NOTE: Files userfileacessor.* will be renamed in a following
commit to settingsacessor.*. It's not done right now because I
think it will make the review easier on gerrit.

Reviewed-by: Tobias Hunger
Change-Id: I950c813936afb6c55c770e0d6f061eb27b37c47b
Reviewed-on: http://codereview.qt-project.org/5081


Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>