Using more *::Constants::ICON_* where it makes sense and
wrapping the file names into QLatin1String where they were
missing.
The increased usage of the ICON constants needed a few more
cross plugin includes of *constants.h, here and there.
I think that it is OK, since the dependencies were alredy there
icon resource wise.
Two kinds of warnings are now shown: one for warning about the Live
Preview itself, as it is experimental, and another when user changes
id's of objects during runtime, as this is not supported at the moment.
More warnings will be added later for other unsupported changes.
