Fixed a Bug in the PaintBrush class.
Fixed up the Image Invert Effect.

- Alpha is now correctly preserved.
- Commented out all the things.
- Did some (?unnecessary?) Pre-mature Optimization.
Added new Effect: RemoveChannels

This effect opens up a dialog with cechboxes. The checkboxes are for
each color channel that the Image has. Checking a channel will remove
the channel from the image (Set it to 0), once the action button is
pressed.
(added icons for new effects)
Added 'Reset'-Button to the Color-Info.
Added notes.

The notes tell where we will have to make changes for the layer-system.
I also added a big note-block at the beginning of the class, in which I
describe how the layer-system -should- work.