More accessibility fixes for Files

More to come … :)

* fix alt text for breadcrumbs home icon
* properly name form labels for the file selection
* add text to Delete action
* add empty alt text for user image	
* fix Delete action text being translatable
* add alt text to 'Cancel upload' button
* add text to Settings button

Please review @owncloud/designers @jerrac @schoeppi @lotophage
I’ll replace the »hidden« for these instances with a proper new class hidden-visually. Cause indeed the current »hidden« class hides it for screen readers as well.