Add a "Color selected" button to annotation list panel

[arjunrajlab]

It would be nice to be able to change the colors of selected annotations (much requested feature). I think adding a "Color selected" button here would be good:

It would just bring up a color picker dialog and assign the colors to the selected annotations. Also, it might be good to have a "Clear color" option in the dialog to remove the color property so that the annotation goes back to inheriting the color from the layer.