Set given color as initial color in gtk color picker

[ttytm]

• Sets the given color as initial color. Keeps the given value if the users doesn't change the color dialog. gtk2 Example when passing an initial color of r: 200,g: 255, b: 255, a: 100:

  Currently it's always: When the user chooses okay, the color r: 255, g: 255, b: 255, a: 255 is returned, no matter the initial input.

  Now, it opens with the values that are passed initially: and keeps them when confirming the dialog without further changes.

  This would resemble the behavior of the color picker windows.

[ttytm]

@AndrewBelt if you find a free moment and it's not too much of a hassle, can you verify the fix for opening the dialog with gtk?

[AndrewBelt]

Thanks! Fixed the color conversion for GTK 2. Rebasing so GitHub will think this PR was rejected.