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.

Also removed a superfluous elide mode setter.

Reviewed-by: kh1

Reviewed-by: ck

Since we use a two column model/treeview we need to prevent returning
some text data, otherwise we will see dots in the secound column. We also
need to emit pagesChanged once we init the model.

Based on the implementation done by ck, slightly modified and thus put
under the private help internal namespace.