[Backgrounds] add support for color drops

[diver]

The Backgrounds preferences should accept color drops (from Colors! or other similar apps) just like the desktop background itself does.

[Giova84]

If I correctly understand, currently color drops works fine on Haiku: eg If I drop a color from the application "Colors!" to the Desktop, this will change the background color. However if i do this operation while the Background preflet is open, the preview isn't updated.

[pulkomandy]

Added a description to the ticket so it is more clear :)

[TwoFx]

I wrote a patch that should address everything that was discussed here.

[korli]

I think that BackgroundsApplication::MessageReceived() should only forward the dropped message if it contains a "RGBColor" data of type B_RGB_COLOR_TYPE. One could drop something else and be handled later in the method.

[TwoFx]

Replying to korli:

I think that BackgroundsApplication::MessageReceived() should only forward the dropped message if it contains a "RGBColor" data of type B_RGB_COLOR_TYPE. One could drop something else and be handled later in the method.

You are right.

[korli]

Patch applied in rev48589. Thanks!