Both implementations are now split into two source files, based on the
base class type. They share also a commen implementation, but the class
interface looks now clean to the outside. Adopt other classes for use.

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.

Some categories do not have icons yet, unfortunately.

Reviewed-by: kh1

We now do as much as possible lazy initialisation, e.g only start the
full text search indexer if we really access the search widget. Also
moved the QFutur progress into the search widget. Changed all ctors to
be called without an argument, since we share the help engines from
help manager all over the place. Make use of the fact that we only need
to call setupData on the gui engine if we access the help mode, otherwise
all data is fetched thru the core engine whichs setupData is really fast.

Currently we can only retrieve a list of keywords from the help index
model, which is slow and needs a fully setup gui help engine. This needs
to be implmeneted in the help lib though.

Add some more statics that can return the class instance,
implements a gui help engine and the bookmark manager for
easier sharing.

Reuse the core help engine in the settings pages. Do not setup the gui
help engine at all since any getter will do so. Split update documentation
into smaller pieces for better performance.

Reviewed-by: ck

Reviewed-by: ck

Reviewed-by: ck

Reviewed-by: ck

Reviewed-by: ck

Go to About Plugins and enable/disable plugins from there.

Reviewed-by: mae

Reviewed-by: kh1

We need to append "?view=split"to force the split mode check. Make the
check more readable in viewerForContextMode.

In case of split mode we did not check if the actual requested doc
exists and thus did fail to open the browser if it could not be found.

Reviewed-by: Daniel Molkentin

Reviewed-by: ck

the one run of the indexer.

Reviewed-by: ck

After this change and the change we did to 4.7, we are down to ~100ms
time in extensionsInitialized on Windows, should be even more on Linux.

Reviewed-by: ck

Reviewed-by: ck