doc: improved some README.xml wording
adg: protect autoscale() from NULL scales

When AdgCanvas:scales is NULL, adg_canvas_autoscale() was crashing
because of NULL deferencing. This was exposed by the LGI bindings but
not by the adg-demo.c program because adg_canvas_new() set the scales
property to a not-NULL value while the bindings directly use
g_object_new().
adg: provide a default value for AdgCanvas:scales

Instead of defining the default value of scales in adg_canvas_new(),
define it in adg_canvas_init(), so also when constructing the object
with g_object_new() that property is preset.
adg: corrected name convention for AdgProjectionScheme

LGI bindings are quite picky in following GLib conventions, so
AdgProjectionScheme values must be prefixed by ADG_PROJECTION_SCHEME_.
adg: renamed AdgMixMethod to AdgMix

The enum values were missing METHOD anyway, so changed the type name
accordingly. Also, the AdgEntity:local-method property has been renamed
to AdgEntity:local-mix.
adg: properly initialize AdgArrow:local-mix

AdgArrow must use a custom default value for local-mix: set it properly
also when using g_object_new().
adg: properly initialize AdgText:local-mix

AdgText must use a custom default value for local-mix: set it properly
also when using g_object_new().
adg: properly initialize AdgToyText:local-mix

AdgToyText must use a custom default value for local-mix: set it properly
also when using g_object_new().
adg: properly initialize AdgTable:local-mix

AdgTable must use a custom default value for local-mix: set it properly
also when using g_object_new().
adg: ensure local-mix in AdgTitleBlock behaves like in AdgTable

AdgTitleBlock is a special AdgTable, so they should mix local matrices
in the same way. The provided test case is not that needed because
AdgTitleBlock inherits from AdgTable but previously g_object_new() was
setting the local-mix default, so it has been included to avoid
regression in such case.
doc: updated it.po
adg: corrected AdgToyText test unit

Copy and paste error from AdgText test unit: corrected and properly
enabled.
build: adg-canvas.h is generated by configure

The header file adg-canvas.h is autogenerated by configure from
adg-canvas.h.in, so it is found in $builddir, not in $srcdir. Handle it
accordingly, changing "adg-canvas.h" to <adg-canvas.h>.