Remove unnecessary computation in -[PXSourceListDelegateDataSourceProxy methodSignatureForSelector:].

Fast-path delegate and data source methods (those whose method
signature doesn't need modification) don't need to be checked in
-methodSignatureForSelector: because they are handled in
-forwardingTargetForSelector: which bypasses the
-methodSignatureForSelector:/forwardInvocation: path.
Fix #40: Editing titles on cell based source list causes exception.

The cause of this bug was returning YES in
-[PXSourceListDelegateDataSourceProxy respondsToSelector:] for the
NSControl delegate methods -controlTextDidEndEditing:,
-controlTextDidBeginEditing: and -controlTextDidChange: when they were
called on the proxy because NSOutlineView implements them internally.
However we weren't returning a method signature for them in -methodSignatureForSelector:
which was throwing an exception.

This fix has two components:
- We only allow forwarding of NSOutlineView(Delegate|DataSource) methods
  to the source list PXSourceListDelegateDataSourceProxy (which
  was the original intention). If PXSourceList returns YES for
  -respondsToSelector: we ignore it if the method is not from one of
  these two protocols.
- The NSControl delegate methods have been added to the fast-path
  forwarding delegate methods array in
  PXSourceListDelegateDataSourceProxy (the array which contains method
  names which can be forwarded to the source list's delegate as-is, without
  modifying the selector or arguments).

These fix the underlying cause of the exception and implement
the missing behaviour of allowing invocation of these NSControl methods
on the source list's delegate.
Fix issue in view-based source list example.

This fixes an issue where items created with the add button couldn't be
dragged.
Bump project to 2.0.3.
Add Release Notes for 2.0.3.