Adding media to post should be done on background thread

[tonyr59h]

From #2317

For every media item selected by the user a WPImageSpan is added to the post. This work is done serially and on the main thread, if there are too many items selected the editor will hang until all the spans are generated. The bitmaps for the WPImageSpans should be generated on a background thread to reduce user impact.

See video

[tonyr59h]

#2515 improved the situation but it will still be noticeable when the user selects more than ~15.