Implementing accessibility non-regression check tool

Hello,

This is a small mail to briefly present the work we (Hypra) will be
doing for accessibility in LibreOffice.

Basically, the idea is to design a tool which will check .ui files for
accessibility issues: missing relations between widgets and labels,
notably. The tool would just use libxml to parse the files and emit
warnings for the found issues.

Such a tool could be called by the LibreOffice build system so that
developers get the warnings along other compiler warnings, and treated
the same way. It could also be used in Continuous Integration reports.

Of course, there are a lot of existing issues, so we plan to add support
for suppression rules, so that when the tool invocation is integrated,
we also integrate an initial set of suppression rules which allows to
start with a zero-warning state, and then for a start developers will
try to stay without warning, and progressively fix existing issues and
their corresponding suppression rules.

One of the remaining questions we have (it's not blocking for our
immediate development, though) is whether this tool should be integrated
within libreoffice, or within glade. The latter would both allow more
widespread use of the tool by other projects, and make the maintenance
happen there, thus less work for LibreOffice :)

We can discuss about it at the ESC call today.

Samuel