Manage globally installed dependencies

[gossi]

Composer can install global dependencies. composer global require 'pack/age' and composer global update. Create a preference page to manage them, which supports:

• The require section (externalize the section, same as in the composer editor)
• A search section (see #38)
• A package section (see #104)

Why? Later give other eclipse plugins the ability to use binaries that are installed via composer (e.g. phpunit).