[Lazarus] New XML format for project info files - disabled by default

[Ondrej Pokorny]

Hello Juha & Werner,

I see you disabled the new item format for project info files by default 
on Werner's request. I understand that the first version wasn't ideal - 
if you needed to open a project in 2.1 and 2.0/1.8 you always had to 
check the compatibility mode when opened in 2.1.

But the current version is not ideal either - most people do not open 
projects in legacy versions and they have to disable compatibility mode 
for every new project manually.

I came up with a reasonable solution:
1.) set CompatibilityMode automatically for legacy projects.
2.) disable CompatibilityMode for new projects.

That should be good for everybody - the CompatibilityMode flag doesn't 
get lost if you open&save a project in a legacy Lazarus version and new 
projects use the new format.

See https://bugs.freepascal.org/view.php?id=35262

Best
Ondrej