Unify behaviour when clicking on entity/asset references

[yaustar]

Related forum thread: https://forum.playcanvas.com/t/user-experience-glitch/19172/6

When clicking on an Asset Reference in the inspector will select the asset and the details will be shown in the inspector.

With an Entity reference, the Editor changes to a 'picker mode' which has confused some users and they end up accidentally changing the entity reference because they wanted to 'inspect' another entity.

See video below:

https://user-images.githubusercontent.com/16639049/112984759-30181900-9157-11eb-9dad-712e09214d52.mp4

Changing the Entity reference to be the same as the Asset Reference where clicking the reference would select the asset/entity being referenced and add a pencil icon to change the reference would be the way to go I think

[yaustar]

@jiajasper Another one for UX