Vector api

This PR bring the vector-api branch back into master. It's our goal that our next realease of RE3 will contain the openlayers/vector-api branch.

 It contains the latest ol3/vector-api changes. See https://github.com/geoadmin/mf-geoadmin3/commit/e5ac163e1633b86f22237d8c22b4738eb0276488

- As pre-caution, current master has been branched out to another branch https://github.com/geoadmin/mf-geoadmin3/tree/master-pre-vector-api
- vector-api was rebased on master after that
- vector-api contains the hand-picked commits pointed out by @oterral from vector-api-kml2
-- https://github.com/geoadmin/mf-geoadmin3/commit/b0da4b71dc581d7f050716c8e0d4906a17fc1d35
-- https://github.com/geoadmin/mf-geoadmin3/commit/e77d99a7fce421e412a3689db2668fe97f31530f
-- https://github.com/geoadmin/mf-geoadmin3/commit/ffca40d3a5db60e8b38dafa76a58d100f49f6b6e
-- https://github.com/geoadmin/mf-geoadmin3/commit/71cb45893d0abecfa90dcb17340310366f8a122c

While this PR has been tested reasonably and most functionality is there, there are a couple of things to do once it is merged into master:

- Printing integration needs to be finished (all needed stuff should be in ol3/vector-api)
- For KML integration, I let @oterral test.
- There's a known issue in feature tree directive because of changes in ol3 related to dragbox drawing.
- Note also, that on ol3/vector-api, the dragzoom default style drawing changed. It now looks more like RE2. It should be easy to adapt this and apply our own wished style.