Add icons for rule editing screen

This is reminiscent of #399896: Improve UI icons and #533132-7: Rules usability issues. Thank you, Fubik, for continuing work on icon metaphors! I'll try to find some icons to fill in the '?'s below. In the meantime, I'm guessing there'll need to be a similar issue to #1655534: Add classes to links in overview pages to help theming.

'edit' | done
'delete' | done

'add action' | 6.x action icon
'add condition' | 6.x condition icon
'add or' | ?
'add and' | ?
'add loop' | ?

from Conditional Rules:
'add conditional' | same as 'add condition'?
'add if' | ?
'add else' | ?
(later: 'add switch', 'add case', 'add default case', and 'add while').

from Views Rules:
'add views loop' | ?