allow for ol.Object to have symbolic properties

[bkuster]

The API of set, get and listen supports Symbol to be a key. Listeners for Symbol properties use Symbol.toString() for explicit property changes.

closes #7757

To be discussed:

• is the test for support of Object.getOwnPropertySymbols necessary or is this polyfilled?
• assign only works if Object.assign is a function, not for the provided polyfill (symbols are not part of for-in loops). This was ignored since browsers with Symbol support also have Object.assign (aka all but IE11). Ref: MDN assign and MDN Symbol.
• missing IE11 support of current builds: with the move to for-in loops, const declaration in IE11 throws an error. See kangax.github.com.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.