Add support to add sources in the Feature Editor

[HannesWell]

As suggested in eclipse-platform/eclipse.platform.releng.aggregator#2777 (comment), it can be useful to have sources of included and required plugins and features added to a feature automatically, without the need to list them all individually.

This issue is about adding support in the Feature-Editor for that by adding a new check-box in the Feature content section

which, if checked adds an add-sources="true" attribute to the root feature element:

Tools like Tycho, that build the feature, then have to include the corresponding source Plug-ins and Features for all included Plug-ins and Features and should require the corresponding source Plug-ins and Features for all required Plug-ins and Features.
When implementing the tool support one should also check if P2 needs to be updated too or if Tycho is sufficient.