Per-channel values of the pixel color are now displayed in the status
bar when the PickerTool is selected. This information obviously updates
on mousemove.
Now the user can hold primary and/or secondary mouse buttons
and move the mouse around while previewing the color on the
statusbar and fine tune their selection. The color will update
live so the color selected when mouse is released is the final
color used.
Refactored PickerTool functionality to the ImageEditor level and added a
flag to Tool Base Class to allow for tools to override Alt+Click
ColorPicker functionality
This is in preparation to support masking of Layers. We now distinguish
between the "display_bitmap" which will be the whole Layer with every
effect applied and the "content_bitmap" which contains the actual
unmodified pixels in the Layer.