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

Fixes: Random enabling of submit/diff buttons when switching contexts (viewing diffs) from the submit editor

Details: Register actions directly

It didn't make sure its Function* stayed valid. Now it does, by keeping
a copy of the list of CppDocuments (the snapshot) around.