ADG canvas 0.9.4 released

* Allow to build the ADG project with meson (https://mesonbuild.com).
  This is a major feature that allows to modernize part of the codebase
  and speed up the building process. The legacy autotools code will be
  kept around for some time, but meson is the preferred one.
* Drop Travis code integration and switch to GitLab. Thanks for the
  fish Travis, but the new model is not that opensource friendly.
* New API (adg_canvas_export_data) that returns the data as a memory
  chunk instead of writing it to a file. This is a requirement for
  adg-openresty (https://github.com/ntd/adg-openresty).
* Update documentation building to latest gtk-doc changes.
* Improve uninstalled detection at runtime: the adg-demo-uninstalled
  hack is no longer needed.
* Fix g_memdup vulnerability.

build: bump version to 0.9.3

build: bump version to 0.9.2

build: bump version to 0.9.1

build: bump version to 0.9.0

doc: update DOAP metadata

Page margins and paddings are now managed consistently across

the different surfaces and their behavior has been documented.

The NSIS script for generating the Windows installers has been
subject to many improvements: it can now optionally install
ADG and CPML manuals (either in HTML and in PDF format) and the
localization data (only italian for now). It should also handle
the application icons properly.

The adg_canvas_export() function can save a canvas to a file in
one command. It needs a patched gobject-introspection though: see
bug #743364 (https://bugzilla.gnome.org/show_bug.cgi?id=743364).

The localization infrastructure has been improved up to the point
gettextize and intltool are no more required.

The algorithm to use for offsetting Bézier curves is now selectable by

calling the non-reentrant cpml_curve_offset_algorithm() function.

A new offsetting algorithm (BAIOCA) has been implemented. It tries to
minimizy the error between the offset curve and the ideal offset point
on evenly spaced t values.

The source code has been revamped to minimize the warnings raised by new
versions of gcc, gtk-doc and gobject-introspection.

The internal sources have been refactored to manage GTK+2 and GTK+3
differences with the same codebase.

Improved documentation: the API reference manual is now kept in

sync with the online documentation (http://adg.entidi.com/) by
using a dedicated SilverStripe module (http://gtkdoc.entidi.com/).
This really simplifies the maintenance burden of the doc.

A bug that was preventing the correct finalization of the returned
cairo path from GObject introspection bindings has been corrected.

The compatibility of the project with outdated system has been improved

in order to be able to install it on old web servers.

Compilation without GTK+ support has been tested: the canvas is now
properly working on a server without X server installed.