* The minimum GIMP version for this XCF is always written down when it
is GIMP 2.8 or over.
* The list of features warrating the minimum version is now listed in an
expander container rather than as tooltip (making the list actually
discoverable!).
* The warning on compression is now displayed as its own text under the
checkbox (only when checking the box actually changes anything
compatibility-wise), and not as additional text to the minimum GIMP
version label. Otherwise it had far too much weight on the minimum
compatible GIMP version text and people were wondering what it meant.
We should not tell people what the checkbox cannot do (it cannot
improve compatibility in some cases), only what it can do (it can
improve compatibility in other cases).
* Update the feature list real-time when checking the compression box
(making it even more obvious that tile compression may have a say in
compatibility).
* Metadata info is still added on the GIMP version label as it does not
limit compatibility of the XCF file itself.
(cherry picked from commit 573d817539)
Both in the GimpImage API and in the GUI. The toggle in the save
dialog now controls ZLIB compression directly. Changed the various
info labels accordingly. Ditch the XCF parasite that saved the XCF
compat mode.
The code refactoring allows better readability which will allow in turn
to add more complex specific features making good use of the save/export
split concept.
