[Lazarus] ProjectGroups: fixed LPG XML structure

[Ondrej Pokorny]

Hello,

I just fixed the XML structure of LPG project group files. Old files 
will be loaded correctly but the new LPG file cannot be opened in a 
legacy Lazarus installation.

AFAIK project groups are usable only in trunk so this does not matter 
(?) Otherwise we would need some legacy option checkbox like it is in 
the LPI project files.

Ondrej