make only one po file per language

make only one po file per language

instead of creating a po file per source package
(that are: pmg-gui, proxmox-widget-toolkit and pve-manager),
but combine the messages from them and create one
po file per language.

to not include unneeded strings in the (pve|pmg)-lang-*.js files,
we use the references value in po2js.pl to filter out unwanted strings

also include a git submodule update in the 'update' target, so that
we get the newest version when updating the po file