Fix issue when extensions are calling inkscape
Fixes an issue described in the following issues
In short, extensions which use inkex.command.call to call inkscape itself receive this error message terminate called after throwing an instance of 'Gio::Error' in Windows and Linux. This can be fixed within inkex (extensions!589 (closed)) but if the extension authors decides not to use inkex, they would run into the same issue again when they do not set the self_call variable themselves. So it'd be better if this was fixed in inkscape directly as mentioned by @mgmax
Additionally this sets a new app id in case extensions want to run inkscape with a gui, otherwise they receive a timeout which is caused by conflicts with the previous instance.