Some changes to RNA property/struct comparison.

Could very easily get into infinite loop when comparing Pointer
properties... And in any case, comparing ID pointer should be enough, no
need to dive into all ID properties!

Moar WIP: add operator to make override of linked object.

Mostly nasty testing code, only overriding location of object for now.
And of course, we are still missing bits in read/write code to actually
do the override.

Also, add some minimal exposure of override in RNA, here again, most to
be done still.

More WIP: add basic new BKE and RNA override API, update DNA structs.

Still much to do to even get a first dead simple working prototype, not
doing anything yet (except compiling and not crashing when ran :P ).

Some cleanup and refactoring.

Final bits to get first (very raw, limited and experimental) override working.

Mostly code using override data to selectively update a copy of linked one
with local data.

As said, still totally hackish and experimental, but it works - you can
create overrides of linked Objects, and override their position, yeha! :P

Some initial, WIP DNA/RNA/readwrite code for static ID override.

Quite obviously totally useless (and harmless) currently, compiles but
does nothing.

Revert initial hack to get a new overridable RNA prop flag.

Now that we have some new free ones...

Quick and dirty hack to get overridable RNA prop flag.

To be reverted once D2400 is committed.