Customizer file uploader should indicate when an upload is happening

[cliffseal]

When dragging an image into the 'Drop a file here or...' area in the customizer, nothing indicates an image is being uploaded. That's confusing, and can lead to multiple uploads of the same file—especially if they're larger files on a slower server.

We should change 'Drop a file here or...' to 'Uploading...' or an AJAX spinner while the image is being uploaded.

[westonruter]

We could utilize the new processing state in 3.9 to indicate when the spinner should be displayed. This is used in Widget Customizer to prevent saving the settings before the Widget logic can return with the sanitized widget instances. We could also increment the processing state when an image upload is happening, and show a spinner when processing is not 0.

[cliffseal]

Here's an example to get us started. I'd love to see a more elegant way to go for it, but this shows how easy it could be to accomplish this extra little piece for users.

[cliffseal]

Conversely, we can plug these calls into customize-controls.js, which is probably a better decision as far as scope goes. 27454-2.diff is a demonstration of this.

[westonruter]

I suggest that we close this in favor of #21483, which eliminate the current upload control for background images and instead handle uploading inside of the Media Manager modal. All uploads can be done via the Media Manager.

[cliffseal]

@westonruter Agreed, though I saw in that thread that the idea was being tossed around to keep the upload drop zone in addition to the button to open the library. If the dropzone stays, we should an an 'uploading' indicator.

[nacin]

Duplicate of #21483.

I like the idea of a dropzone that opens the media manager. That's about the extent of which I'd like this control to remain.

Worth remembering that the customizer (3.4) came before the media manager (3.5). We definitely would have done the customizer differently if it were the other way around.

[cliffseal]

Replying to nacin:

I like the idea of a dropzone that opens the media manager. That's about the extent of which I'd like this control to remain.

Right on. That's an indication the upload is happening just as well.

...almost got some code into core this time, though! :D