Related: xfce/xfce4-dev-tools#41

For consistency with other components and for the GLib macros set by
XDT_CHECK_PACKAGE when building with xfce4-dev-tools >= 4.17.1.

* Use git version tag
* Update project url

- Detect the presence of Python 3.7 and error out correctly if it is not
  available
- Enforce using the correct Python executable for generator.py

* Bump required version to 2.69
* Replace some obsolete macros

- Switch debian build compatibility to 9
- Update dependency on xfce4panel to 4.12
- Use libxfce4panel-2.0 instead of 1.0 for build.