Allow out-of-sources build by setting Make variable srcdir.
This still generates the scintilla.a library in the source directory, though.

Currently, the ScintillaObject signals can't be used from python code via
gobject-introspection. This is because g-ir-scanner does not properly scan
the signals.

For signals, there is the additional requirement that parameters have are
registered GTypes. For the sci-notify signal, this can be accomplished by
boxing SCNotification.

In addition, g-ir-scanner also runs on Scintilla.h where it picks up
additional structs.

test/gi/ is updated accordingly. The test python script is enhanced
to showcase the signals (some fixes to the makefile are included as well).

This makes it easier to update or add new marshallers.

Also, re-generate the current one using a current version of
glib-genmarshal, which generates different output (accessing different
fields for enum and flags) since at least 2008.  Although neither of
these types are currently used, it seems safer this way.