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
* add label for search field for screen readers

Please review @owncloud/designers @jerrac @schoeppi @lotophage @henningoschwald