Scratch-vm: Make the color picker input non-droppable

[jwzimmer]

Expected Behavior

You should not be able to drop into the color picker swatch (but we should continue to support 2.0 projects that use a hex value). /cc @ericrosenbaum for details.

Actual Behavior

Dropping a block onto set color block with color picker causes an error.

Steps to Reproduce

Drop a block into the color picker input.

Operating System and Browser

_Mac, Chrome, https://ericrosenbaum.github.io/scratch-gui/extensions-oct2017/_

[mrjacobbloom]

Why not? There are use cases for setting pen color to a number. All of the projects in https://scratch.mit.edu/studios/1440810/ draw an image by pulling pixel data from a list and setting the pen to that color. Storing/drawing images in this way would be far harder if you had to set hue/saturation/brightness manually

[Kenny2github]

Inputting a hex color in 2.0 is as simple as this:

set pen color to ( (join [0x] [ffffff] ) + (0) )

since "0xffffff" + 0 = 16777215 and the "set pen color to [color]" accepted reporters that reported a decimal color.
I think this behavior should be duplicated in 3.0.