Move stand-alone OL functions to custom ES6 modules

[probins]

The first stage of converting OL to ES6.

• DOM elements:
  • controls
  • Overlay
• other simple objects:
  • coordinate
  • extent
  • Sphere
  • formats?
  • loadingstrategy.bbox?
  • OSM?
• split formats into separate read/write modules?
• fork Map or add baseMap? (will be fixed upstream)
• replace getxxx() with get('xxx')