Add "Switch To Buddy" plugin.

This enables in the C++ language plugin to add a new context menu
action to switch to corresponding header or implementation file.

Changes made:
- A new function has been added to IBuddyDocumentFinder to allow
language plugins to implement logic to find buddy or related files.
- The new plugin "switch to buddy" implements a context menu extension
which is only trigged if at least one buddy can be found. To get
possible buddies IBuddyDocumentFinder is used using the editor's
current document's mime type.
- See plugin

This implements a feature known from many IDEs. Using the Buddy
interface and a separate plugin I think it's rather clean and generic.

REVIEW: 107310