generator: Fix indentation in Signal.cs

Only whitespace changes, no real code change.
generator: Dispose ownable parameters in signal callbacks (bxc#237)

A similar situation to what is described in commit e48ac63d54c4 also
happens with signal callbacks: some signals are passed a native object
that is wrapped in an IDisposable managed object, which is then passed
as an argument to the signal handler. We need to dispose those objects
when the signal handler is done.

Those parameters will now be disposed in a finally {...} block, after
the signal handler has returned. This means that handlers should not
keep a reference to such a parameter, as it will be disposed right after
they return.

This change only affects the Cairo.Context parameter of the Widget.Drawn
signal, but it was badly needed, as shown by the Pixbuf demo in the
GtkDemo sample, which was leaking tens of MBs of memory.