Rename #getImageElement to #getImage

This adds consistency between ol.Image and ol.ImageTile. Now both have
an exportable getImage method.
Give ol.Image an imageLoadFunction argument
Add a default imageLoadFunction to be used by sources
Add imageLoadFunction option to ol.source.ImageMapGuide
Add imageLoadFunction option for ol.source.ImageStatic
Add imageLoadFunction option for ol.source.ImageWMS
Add missing types to olx.js
Merge pull request #2891 from ahocevar/imageloadfunction

Introduce a load function for image sources