gobject introspection wants a capital letter for the gir file

• See https://gi.readthedocs.io/en/latest/buildsystems/autotoolsintegration.html: "Namespace of the gir, first letter capital, rest should be lower case"
• This will help vapigen to find correct namespaces
• Update m4/introspection.m4 from upstream
• Panel now requires libxfce4util 4.15.6