Project: Allow for additional named settings

Allow for additional named settings to be saved along with the project
data.

This allows plugins to save project-specific settings.

Change-Id: I6ed24089efad2eb466385ac9ca4c2dde8bf8c2eb
Reviewed-on: http://codereview.qt.nokia.com/2443


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>