Actor: Made register_actions purely virtual.

This ensures Actor implementations will have to implement it. Even if
empty.
History Book: Better comment.
UI: Added call action to main window + UI fixes.

This adds a call button to the main window UI. Actions being
added/removed on the fly depending on the context (e.g. if a contact is
clicked), the button will become sensitive/unsensitive automatically
depending on the Action availability.

This is especially useful for global actions like "call".

The patch also fixes buttons alignment to be more conform to what we
would expect from GNOME programs.
Engine+OPAL: Added new global actions.

Global actions were added to implement the minimum functions we would
expect from a softphone:
- Call (coming from the CallCore)
- Transfer (coming from the CallCore)
- IM (coming from the CallCore, but should probably be moved in the
  ChatCore)

The corresponding methods were added to the Core and to the respective
CallManager/CallProtocolManager classes because they are the minimal
subset of operations we want endpoints to implement. More specific
actions will be exposed to the Actor mechanism too, but will not be part
of the Core API.

The old populate_menu stuff has been (partially) removed.
Notify: Added missing include.