Blender Git Loki

UI: Use different icon to remove user-created shortcuts in Preferences

There was no way for users to visually tell appart shortcut items
they've added themselves and the default ones. Both used the same 'x'
icon to remove the item.
Modified items already showed a back-arrow icon; this commit makes
user-created shortcuts use an icon that has both, a back-arrow and a
'x'.

The icon is in fact from the Movie Clip Editor, but William Reynish and
I concluded that it's not worth creating and adding a new one just to
short-term fix this small annoyance. And the icon actually fits the
purpose surprisingly well :) {F8485176}