The mouse handling for code folding wasn't deactivated when the code
folding markers were hidden.

Reviewed-by: mae

RevBy: dt

RevBy:    con, hjk

location was removed from an editor

in ifdefed out sections

in the right editor from the start

Done based on new IQuickOpenFilter property, isHidden.

This change removes a number of wrong usages of the CppModelManager::documents()/document().

Task:     Reported on the ML multiple times.
Details:  Add a few methods to make it possible to save and restore
settings. Save those settings to the .ini file. (I'm not sure whether
seesion would be better, let's see what feedback we get.)

when opening files from the C++ editor (e.g. when the user press F2 on #include <QMutex>).

support statements later.