Implement Image-processing

[t0815]

Implement imageprocessing logic:

• [x] General logic
• [x] image conversion
• [x] image rotation
• [x] image resizing
  • [x] Fix resize bug (w was set as height too)
  • [x] Add lock/keep aspect-ratio option
  • [x] Add resize percentage option
  • [x] Add dont enlarge/stretch option
  • [x] resize to specific page
• [ ] page splitting
  • [x] split at percentage
  • [x] split at fixed length
  • [ ] detect split position from color
• [x] impl. task-order selection
• [x] add missing settings #18
• [x] update thumbnail (maybe even add an eventhandler to trigger updating thunbnails...)