QmlDesigner: remove DesignDocument::goIntoSelectedComponent()

This function is highly confusing as a member of DesignDocument
and the logic can be implemented directly in the ShortcutManager.

Change-Id: Id722dcb71b542b4780614c9f895f06150ccb67ca
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner: remove DesignDocument::changeToExternalSubComponent()

This function did nothing but call EditorManager::openEditor(), so we can
call it directly instead. It was not used consistently anyway.

Change-Id: I639ef346164be9260e887faac5dc6651ccfd92c5
Reviewed-by: Marco Bubke <marco.bubke@digia.com>