Download java sources lazily

[fbricon]

As described in #824 (comment), eagerly downloading sources causes serious performance issues when working with large projects.
@snjeza prototyped lazily downloading sources in SourceContentProvider, as described in

8 - Java LS downloads all maven sources artifacts. Che 7 uses an empty maven repository when importing a project. I have added a feature that enables downloading a source code when it is required.

This is a good starting point but we need to address a couple points:

• SourceContentProvider should be decoupled from m2e, so we could probably either introduce a new extension point to hide the source download mechanism or bind it to the generic IBuildSupport
• Sources were eagerly downloaded to provide a good UX when hovering over 3rd party classes. So the lazily download should also be triggered on hover to maintain a close behavior.

[fbricon]

Best place to lazily trigger source download on hover would be in HoverInfoProvider IMHO. Need to check that element.getOpenable().getBuffer() == null and we have already tried to perform a download (so attempts should be cached/recorded)