Add a better pointer/message in the new media UI with an empty library

[apeatling]

The new media UI is a little stark when you have no media in your library. It wasn't immediately obvious where I could add new files, I had to spend a few seconds scanning the corners looking at each UI element (search, insert button and upload UI) since they all carry the same weight to the eye.

There's a big white space where photos will eventually be, that's where I look first. It would be good to use that space to instantly grab the user's attention and direct their eye towards the upload UI. Don't make them think. :)

I've attached an image of the basic idea.

[apeatling]

My first idea was to add another upload/dropzone section to the white space, but I think it's better to be consistent and associate that bottom left section with uploading from the get-go.

[rzen]

I spent a few hours today patching this while Koop was busy restructuring the entire modal. Ultimately, his new direction (from ​r22362) is a much, much better fit.

The included patch merges in methods from media.controller.Upload to media.controller.Library so that the inline uploader is available basically everywhere (specifically, this is useful for displaying it in the Media Library section when we have no media, e.g. ​http://d.pr/i/bgWO, the original reason for this ticket).

Fist bump to Koop for basically holding my hand through this patch. Excited to be contributing anything, really.

[koopersmith]

In 22363:

Media: Show an inline uploader when the media library is empty.

Merges most of wp.media.controller.Upload into wp.media.controller.Library, so we always create an inline uploader instead of an attachments browser if the state's library is empty.

props rzen, fixes #22323, see #21390.