Summary / How this PR fixes the problem?

Implement the alpha mixing of multiple fill and border colors.

Steps to Test

• Create a shape
• Add multiple colors and fills
• Play with the alpha of the extra colors

Screenshots

Known Issues / Things To Do

The color gets reset once the item is selected, which doesn't make sense. Fixed!
For now we only handle 1 border size, so we get the bigger and go with it.
In the future we will probably implement child items just to handle different border sizes and also different offset once implemented in goocanvas.

This PR fixes/implements the following bugs/features:

• Blend colors by their alpha channel for fills and borders.
• Use the color picker icon for the color picker button.
• Move the color picker close to the color button and style them to make them related.