Add support for code browsing and content assist of Grapes (aka @Grab dependencies)

I'd like to propose to reconsider #236, that is: make some integration of the Grape dependency manager with the Groovy plug-in.

Grape is great and it makes Groovy very practical for writing quick scripts, prototypes, etc. However, it is also very impractical to use it, without some integration with Eclipse.

While I understand the problem of multiple classpaths associated by multiple scripts in the same project, there are reasonable solutions to that, e.g., - merging all the declaration and notifying the user of possible conflicts - manual right-click on a @Grab annotation to include the corresponding dependency to the project classpath.

As it stands now, the Groovy plug-in is of little use to me, without this feature (and consequently, so is Groovy).

Start by reviewing the related issue #236 and the Groovy plug-in's existing handling of Grape and @Grab dependencies. Determine how code browsing and content assist should account for dependencies declared by multiple scripts, including the proposed conflict and manual-inclusion approaches. Done means the Eclipse integration supports the requested dependency-aware browsing and assist behavior.