Set the default projection color to the selected color

CCMAIL:kimageshop@kde.org
BUG:340216

We used to set the default pixel color of the first layer to the color
selected in the custom image widget. We also have a default projection
color. This change sets the color and transparency set in the custom
image widget as the default projection color.

This has two consequences:

* erase will now no longer show up the transparency checkers if you
use an opaque background color (which is the default)
* in order to have transparent areas in the image, you need to use
the image properties or the new image background color menu option.

Please test and give feedback whether this is what we want, or whether
we need refining.

Two possible issues:

* erase still doesn't erase to the bg color selected in the toolbar;
it will erase to the bg color selected for the image
* loading, say, a jpg and using the eraser will still by default erase
to transparency checkers. We could consider making the default background
color for an image solid white...