This should allow to have various instances of the same extension with the same
identifier but different sets of target platforms.

During the update the extension manager needs to determine the proper update
based on the supported platforms and the current platform. Only if an "update"
supports the current platform then it can be installed. If there are more then
one possible updates (all having the same version but different sets of
supported platforms) then the first found, that supports the current platform is
used.

Reasioning:
1. It is not necessary that a new version supports all of the target platforms
of the installed extension. Assuming a multiplatform extension is installed in a
heterogeneous environment where the user installation is shared among OOo
instances on different machines, then the extension only works on the machine
where it was installed. This is because the libraries are actively registered on
the system. In other words, the libraries are executed during installation.
With a passive registration one could install on one machine an use it from all
other machines right away.

Note: When one wants to install extensions in heterogenous environments then one
should provide different extension identifies otherwise one extension which was
installed on a different platform would be replaced when installing another
instance of the same extension on another machine (same extension identifiers
but different target platforms)

Assuming we had a passive registration mechanism, then we should choose the
update so that it supports at least the same platforms as the original extension.

It can happen that a new version adds support for additional platforms as well
as remove some of them. Therefore the update window needs to display that an
update is available but cannot be installed because it does not support the
platform anymore.