Change callin markers to work with mementos

[stephan]

Originally, callin markers were implemented by storing the respective method mapping in a marker attribute. This was illegal as documented in IMarker.setAttribute(String,Object):

The value must be <code>null</code> or 
an instance of one of the following classes: 
<code>String</code>, <code>Integer</code>, or <code>Boolean</code>.

Since 3.5 Eclipse enforces this rule on all paths, so we can no longer store objects in the attribute, but must use a handle identifier (aka memento) for communicating the mapping from CallinMarkerCreator2 to the UpdateRulerAction.

[stephan]

Changes to the callin marker creator are basically implemented in r19979.

Creating and decoding mementos for method bindings has been implemented in r20086, r20094, r20095 (including basic tests).

The context menu has been fixed in r20097.