These diagrams are not expected to change much, so just include the generated
PNGs in the repository, and remove 'dia' from the prerequisites to build the
documentation.
It's pointless to have libmm-common around, just merge it into libmm-glib and
make ModemManager depend on libmm-glib directly. At the end, the non-common
stuff in libmm-glib is really minimal.
Elements in a DBus interface name cannot start with a digit, so
"org.freedesktop.ModemManager1.Modem.3gpp" is an invalid interface name.
Renamed here all relevant interfaces so that they have an additional "Modem"
prefix in the element, so we get now:
"org.freedesktop.ModemManager1.Modem.Modem3gpp"
"org.freedesktop.ModemManager1.Modem.Modem3gpp.Ussd"
"org.freedesktop.ModemManager1.Modem.ModemCdma"
Objects generated with gdbus-codegen maintain the previous names.