GTK 3.21.0 fixed font size handling, leading to properly interpreting
pixels and points in CSS declarations.  However, as older versions
incorrectly handled those, the code has to handle both behaviours.

From CSS, GTK < 3.21.0 actually applied the conversion to points, but
incorrectly: 10px was used as 10pt, but 10pt was scaled up twice.

So, assuming 96 DPI, it leads to:

font-size | 3.20.0  | 3.21.0  |
----------|---------|---------|
10px      | 13.33px | 10px    |
10pt      | 17.77px | 13.33px |

So, we need to fix the code to accommodate for both (either scaling
ourselves, or adapting the unit: I chose the second, simpler, option).

See https://git.gnome.org/browse/gtk+/commit/?id=df08fc91bdc1d2e4c866122304fabe4dd298a7de

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).