Adjust list of replaced elements to the spec

"button, textarea, select, div[contenteditable='true']" were removed and
"audio, canvas, embed, iframe" were added.

Note that this is a coarse-grained list; according to the spec some of
the elements, like canvas or input, might be treated as a replaced
element in some cases and as ordinary elements in others:
https://html.spec.whatwg.org/multipage/rendering.html#replaced-elements

For now all the elements that might be replaced elements use the loading
image by default, so apps will need to override that when the elements
are treated as ordinary elements. Of course that can be flipped in the
future to instead make an element to use the "::after" approach by
default if it is found that the element requires the override often.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

core/css/icons.scss

@@ -81,7 +81,7 @@
 }
 
 /* Css replaced elements don't have ::after nor ::before */
-img, object, video, button, textarea, input, select, div[contenteditable='true'] {
+audio, canvas, embed, iframe, img, input, object, video {
 	&.icon-loading {
 		background-image: url('../img/loading.gif');
 	}