Switch to tiny's built-in loading animation

[indigoxela]

That code part (Backdrop.tinymce.openDialog) has been (mostly) copied from CKE4, but I don't think that delayed throbber animation thing ever worked.
Even with CKE4 it never appears. Neither with CKE5.

However, while working on tinymce_filelink, I discovered TinyMCE's nice and easy to use loading animation. And it actually works. And prevents additional clicks on editor elements, while ajax dialogs are loading.
Very useful for impatient users with slow internet connections. 😉 Gives some feedback, that something happens in background.

And it's time, anyway, to clean up that copy-paste mess in tinymce-integration.js