Action buttons need icons and/or colours

[selfthinker]

The action buttons need a bit to make it easier to distinguish them from each other. This is my first try (well, my second, actually):

What I don't like about it and still plan to change:

• The icons need to be lighter (some middle-dark grey).
• It would be great if the "enable" button would be a switched on lightbulb (but such an icon is not in the iconset I got the other from). So, it should be transparent in the middle with some beams around it. See also #38.
• I don't like the "delete" icon. Maybe a simple minus (to go with the plus for "download") would do the job. (But, again, such an icon was not part of the set, but should be easy to make.)