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.

on gdb startup)

Now the box will grow larger when files with long filenames are opened.

forward declaration is sufficient.

Also made sure the glossary terms aren't wrapped, since that didn't look
very nice.

For the documentation about the dependencies the screenshots should be
updated again.

Dependencies between projects can now be edited in the Projects mode.
You can simply check for each project the projects that it depends on,
and they will be built first. A message box pops up when you try to
create a circular dependency.

Could use some testing.

Details:  Works.

Task:     235982