Be able to define a Run environment

It would be really handy to be able to specify some environment variables to set when running the application.

for instance:

MY_INSTALL_DIR=/home/myself/instdir
GST_PLUGIN_PATH=${MY_INSTALL_DIR}/lib/gstreamer-1.0${GST_PLUGIN_PATH}
GST_REGISTRY=${GST_REGISTRY:-${MY_INSTALL_DIR}/.gstreamer-1.0/registry}
PATH=${MY_INSTALL_DIR}/bin:${PATH}
PYTHONPATH=${PYTHONPATH}${MY_INSTALL_DIR}/lib64/python2.7/site-packages/:${MY_INSTALL_DIR}/lib/python2.7/site-packages/
XDG_DATA_DIRS=${XDG_DATA_DIRS}${MY_INSTALL_DIR}/share:/usr/share
MC_PROFILE_DIR=${MY_INSTALL_DIR}/share/mission-control/profiles
GI_TYPELIB_PATH=${MY_INSTALL_DIR}/lib/girepository-1.0