Use notifications in upload widget

[Piotr Jasiun]

Upload widget should use notifications instead of console to inform user about progress and warning.

The prefect solution would be to show one progress notification for multiple uploads.

TODO: Remove the console.log from the uploadwidget plugin.

[Piotr Jasiun]

Related: #11461, #11580.

[Piotr Jasiun]

Working on this ticket I found 3 related issues:

• #12896, this branch is based on #12896, because otherwise behavior does not make sense,
• #12901, creating tests it was very weird that upload needs to create two notifications,
• ​git:7155887, notificationUpdate cancel behavior had to be changed, as described in docs and comment.

Changes are done and thanks to notification aggregator all uploads are displayed in one notification (excluding warning and success messages). Changes in ​t/12724.

[Piotrek Koszuliński]

I pushed a rebased ​branch:t/12724 with some small corrections in docs and tests.

However, I spotted a bug. I opened ​http://tests.ckeditor.dev:1030/tests/plugins/uploadwidget/manual/image and dropped a file there. The progress goes to 13% and then I see a success message. The file I use has 74kb (1000/74 == 13).

[Piotr Jasiun]

I pushed a changes in XHR mock so it works fine with every file size.

[Piotrek Koszuliński]

Fixed on major with ​git:22b9485.