Support installing plugins in /usr/local/lib/eclipse and /usr/local/share/eclipse

[Pantelis Koukousoulas]

We need a location for plugin/add-on installs that are both:

       1) Shared by all users on a machine

       2) Not managed / owned by the distro tools

We can do that by using /usr/local/lib/eclipse for arch-dependent plugins and /usr/local/share/eclipse
for arch-independent ones.

In both these locations we should have the following structure:

               * plugins
               * features
               * dropins

plugins/ and features/ should ideally be owned/managed via P2, if this is possible.
But most admins will likely choose the dropins hierarchy since this is much easier to manage at
least for "leaf" plugins (with no reverse dependencies).

e.g.,

             dropins/
             ________ eclemma/
             ___________________ plugins/
             ___________________ features/