Use string values for ol.source.State enum

Many user-facing enums are string values and we document use of the string values rather than more confusing enum reference or the numeric values.
Merge pull request #2247 from pagameba/source-state

Use string values for ol.source.State enum
Add ol.easing to api docs
Merge pull request #2256 from fredj/easing.jsdoc

Add the jsdoc ol.easing namespace