Use TargetPlatform and TPD dsl generator to simplify and speed up the build

## Motivation

currently, the studio uses pom.xml repository definition in order to specify the update site where to look for additional plugin and features.

The plugin dependency resolution is slow due to the huge amount of plugins in the update sites we reference.

## Proposed Solution

Using a definition of .target (ie. targetplatform) allows to restrict the search. The edition problem (require strict version numbers) of this target file can be handled thanks to a nice editor and generator (https://github.com/eclipse-cbi/targetplatform-dsl )

Additionally, this should simplify the various pom.xml

## Proposed Implementation

we can provide a single target project in gemoc-studio repository that all other repositories can use.

example project using this approach:
https://github.com/cea-hpc/NabLab/blob/master/pom.xml (where the name of the xxx.target file is set in the "classifier" tag) to https://github.com/cea-hpc/NabLab/tree/master/releng/fr.cea.nabla.targetplatform (see the readme)