[430778] Add contributing instructions

The file name follows the GitHub convention so that people trying to
create a pull-request from a clone of this repo on GitHub will be
presented its content.

Bug: 430778
Change-Id: I4b526b265f97da9b412478082226d59a31ce8915
[423707] Replace legacy expressions with core or A3 equivalents

This only handles the cases where there is an obvious replacement.

Bug: 423707
Change-Id: I0dfd19d8a62ba667201c5dcc1d3367201dcc7e0a
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[423707] Remove references to the "legacy language" in the doc

The sections about that language (a pre-Eclipse version of Acceleo) are
not part of Sirius, so it makes no sense to talk about a "legacy
language" that is not presented in the doc.

Bug: 423707
Change-Id: Ib47ec2f255bda3febef243bdfa6c5fd1770b4d94
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[cleanup] Reduce code duplication using EMF reflection

The three different hasNonEmpty*NameDefinition() methods were almost
identical except for the label feature they tested. Rewrite them as
simple calls to a generic version which takes the EReference to use as
an argument.

Change-Id: If1974c02c4f4525a2f89a0edf460f4fa54095944
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[cleanup] Remove the broken tests for empty expressions

The tests were language-specific (and for a legacy language whose
support is not in Sirius itself), and completely broken anyway; they
only deals with very specific cases and give the illusion of working
with no actual guarantees.

Change-Id: Ic5e9e0fd69cb278ce2812390d9be4332e96e525e
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[cleanup] Remove dead code

Change-Id: Ie9476e46d7f028baaec05169d2f1f77157369ffd
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>
[cleanup] Replace broken implementation of findDiagramElements()

As mentioned in the code comment, the method is broken (it depends on
the presence of an interpreter for a legacy language not present by
default in Sirius), not used anywhere in the current code base, and
deprecated anyway. The new version has the same behavior but at least is
fast and does not pretend to do something.

The method will be removed when the corresponding metamodel operation is
deleted as part of bug #427247. In the meantime this cleanup at least
removes hard-coded references to a legacy language not supported by
default.

Change-Id: I2702fd1a4e21ad0358d126d0584b06931ee46f65
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>