[Vala] Signal connect syntax

[Jürg Billeter <j bitron ch>]

The syntax for connecting and disconnecting signal handlers using += and
-= has been inherited from the C# syntax. As described in bug 566847
[1], using these operators does not seem appropriate for signals.
Therefore, the syntax has now been deprecated by

    foo.clicked.connect (handler);
    foo.clicked.disconnect (handler);

The old syntax still works and will continue to be supported for the
foreseeable future to not break existing code. A future compiler version
might print a warning when not using --enable-deprecated.

Jürg

[1] http://bugzilla.gnome.org/show_bug.cgi?id=566847