[m2e-dev] Discovery UI added

[Pascal Rapicault <pascal@xxxxxxxxxxxx>]

I've committed Matt's contrib to add the discovery UI (bundle org.eclipse.m2e.discovery). This discovery UI will help the user discover m2e extensions that are available and also install new versions of the embedded Maven.
This contribution also hooks in a few quick fixes that will help the user address the error he gets when importing projects (e.g. missing packaging type, missing configurator, missing plugin execution, etc.)

At this point this mechanism is in place for Eclipse 3.6 and 3.7. We will figure out the 3.5 story later (if any).