Add icons for keyframe actions

I got it working so far, but in the effect stack the icons appear always dark (in the project monitor it works as expected). This needs to be solved before merging, but I don’t know how… (This is the only reason for the "Draft:" state)

Included icons:

• keyframe
• keyframe-disable
• keyframe-add
• keyframe-remove
• keyframe-record
• keyframe-duplicate
• keyframe-next
• keyframe-previous