Add "Add Post Element" to media buttons

[goldenapples]

Includes an "Add Post Element" to the media buttons array, which pops
open the Insert Post Element frame of the media modal.

See #131

props to @ninetienne and @Vaince for code snippets

[goldenapples]

Clicking this button:

will bring up the "Insert Post Element" state of the media modal.

Curious to hear feedback on this. I know there were concerns raised about adding more buttons to the editor, but on the other hand, this has been a very popular request. We have time before the next release to put some thought into the UI, so it would be good to get some community input. I think adding a media button but not an MCE button is a good middle ground between discoverability and clutter.

[danielbachhuber]

Curious to hear feedback on this.

I dig it. 👍 from me.

[mattheu]

I like the button. But I'm not so sure about the UX of having 2 links to the same modal. I think we need to go further and split this out from the existing media modal completely. See #582

[khromov]

Putting this in the media dialogue has always felt a bit awkward, and we've gotten feedback from multiple editors that it's a useless extra click, so +1 this!

[0aveRyan]

Small issue I've encountered, still working on a fix -- The media modal retains its' state on close.

The script works initially to launch into the right view, but if an image or other media element is inserted first, the modal retains state and will return to image, gallery, etc. Apparently WordPress Core intends to retain state (see: https://core.trac.wordpress.org/ticket/27480). I've spent a few minutes attempting to tap into the modal and reset any existing states and force shortcode-ui, but haven't been successful yet.

[0aveRyan]

@goldenapples right before the wp.media.editor.open( editor, options ), if you add this:

wp.media.editor.remove( editor );

The modal will always trigger the right state, even if there's a stored state from Inserting an image or other view.

[goldenapples]

Thanks @0aveRyan - that seems to work perfectly for triggering the correct state of the modal. I think we'll also need to find an appropriate place of resetting the modal state when the shortcode-ui modal is closed; otherwise, after once clicking "Add post element", when the user tries to add media, they'll get the "Insert Post Element" state.

[0aveRyan]

@goldenapples that would require a more invasive change to the callback where Shortcake inserts the shortcode or the close method is triggered.

I agree and personally prefer the state to reset on close / insertion as well... but the discussion on that WP Core ticket indicates they prefer the state is retained, so while it definitely makes sense to unset for this button, think we should get feedback from someone on wiping out state elsewhere before exploring that.

[goldenapples]

I merged in master to fix the build lint error; this is ready for a review/merge now.

[mattheu]

Looks good. Tested and works well.