- updated appdata file to a newer syntax
- created an english appdata template
- the strings are extracted during updatepot function in translation-update.sh
- the multilingual file is created at the end of updatemo in the same script if possible
While this is not the standard name for the files, this makes the
resource handling easier as the PlatformUtils code already handles
lookup for both installed and dev layouts. At least for Linux the
installed name would be /usr/share/locale, so we use that in source
too for now.