RevBy: Discussed with Thorbjorn

Fixes: Documentation - some extra content and adding the Code Editor page as a placeholder for future content

Details:  - Naming convention

Details:  - Preparation for ICommand rename

Details:  - Conform to naming convention

Details:  - Preparation for IActionContainer --> ActionContainer

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.

Details:  - I'm still not too happy with the whole construction but anyway.
The ToolBarActionContainer was never really used anymore (e.g. you
couldn't create one)

RevBy: Thorbjorn

Makes Qt Creator read external editor string from .ini

Details:
ActionManager::registerAction(qaction, id):
  use registerAction(qaction, id, QList<int>() << Core::C_GLOBAL_ID)
ActionManager::addAction/Menu:
  use ActionManager::actionContainer + IActionContainer::addAction/Menu

Details: Refactor the git command to be able to run a batch. Run diff and diff --cached in a batch

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.

Details:  - Step one, step two is ActionManagerInterface -->
ActionManager because it conflicts with our naming conventions