replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()

It was getting a bit cluttered without any grouping of the different
actions. Also, the collapse and expand actions have been added, mainly
for discoverability.

forward declaration is sufficient.

RevBy: Matthias

Details:  - Naming convention

Details:  - Conform to naming convention

The _ is usually used to prefix internal and/or private methods and
members. Having those turn up at the top of the list of suggestions for
completion isn't too useful. Improves the autocompletion for example for
the STL.

Done together with Roberto Raggi.

The previous interface wasn't sufficient to properly support the Apply
button, since it didn't separate applying the changes from the cleanup
phase.

Designer settings pages still need some fixing.

Fixes Apply button causing problems for certain plugins, for example the
QuickOpen.

Details:  - Conform to conventions.

Details:  - To make it conform to naming convention.

Now with commonly used red and green colors, and without the ugly bright
green for locations.

Otherwise it's really hard to read and sometimes you just need to edit
it a bit.

Now reuses the colors set for Type and Keyword for the variables and
functions respectively. Having the categories Variable and Function in
the fonts and color settings was confusing.

Most desktops allow popups to overlap the taskbar. We should allow this
additional space to be used by the completion popup.

Pop it upwards when it would otherwise go below the screen and shift it
to the left when it would otherwise exit the screen on the right.

Details:  in all pluginspecs.

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

Reviewed-by: mae

in ifdefed out sections

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

support statements later.

Doesn't look right, since it causes some background to show through
between the indicator and the scrollbar.

    enum ExtraSelectionKind {
        CurrentLineSelection,
        ParenthesesMatchingSelection,
        CodeWarningsSelection,
        CodeSemanticsSelection,
        OtherSelection,
        NExtraSelectionKinds
    };
    void setExtraSelections(ExtraSelectionKind kind, const QList<QTextEdit::ExtraSelection> &selections);
    QList<QTextEdit::ExtraSelection> extraSelections(ExtraSelectionKind kind) const;

This is mainly for the benefit of the cppmodelmanager.