Toolbar plugin configurable

Briac Pilpré, member of OmegaT team, sent us a set of patches providing a new set of icons and giving the possibility to configure the toolbar in an xml file: it enables to remove icons, to move them, and even to add a new one (provided that you know the correct name in OmegaT's code for the action you want to associate to the button)

These patches are now in the Git but before to publish a new version there is still some work to be done about the integration of other plugins in the toolbar (actually, extra plugins such as cross-lingual concordance correctly integrate to the toolbar but using an algorithm which does not make possible to configure the location, nor the icon to be used). We still work on this problem, but that's the main reason why we do not publish a new version of the toolbar plugin for the moment. In any case, many thanks to him!