Apply waiting and timeout on all assets, not just img, audio and video

[mrxz]

Description:
The asset loading system differentiated between <img>, <audio>, <video> elements and the <a-asset-item> elements. Only the former would be awaited and subject to the timeout, the <a-asset-item> elements would use the normal 'wait till loaded' mechanism of a-node.js.

Besides the timeout not applying to <a-asset-items> it opened the door for race conditions where the timeout could cause a concurrent super.load resulting in two loaded events being emitted. This is the cause behind #5130

This PR moves the <a-asset-item> handling into a-assets ensuring it waits on all assets in the same way. The timeout will now correctly cause the loading event being emitted without awaiting any (pending) assets.

Fixes #5130

Changes proposed:

• Wait on <a-asset-item>s in a-assets
• Avoid race condition where a-assets would emit two loaded events

[dmarcos]

Thanks!