Gb-studio: Make "Set Background Palette" options clearer

[coasterspaul]

Is your feature request related to a problem? Please describe.
In the Palette: Set Background Palette event, leaving a slot as "None":

actually sets it to the scene default. [Thanks to um3k for the clarification.] This is pretty useful for stuff like palette cycling - you don't have to set every single palette in every event - but a lot of people don't seem to know it's there.
(I think some people assumed "None" just meant the DMG palette, which isn't helped by the color preview.)

Describe the solution you'd like
Change the text for "None" to "Scene default" or similar. And change the color preview - either:

• hide it entirely
• set it to all white, which would look the same as getting rid of it entirely in the light theme, and it'd probably still make sense in other themes
• somehow actually get it to show the scene default for that palette slot, if that's even possible

Describe alternatives you've considered
Also, a "Keep As-Is" option, leaving a palette set the same instead of resetting to the default, would be useful for even wilder color-changing, assuming that's possible with how the event's set up. Since there'd be no way to tell what colors that would be ahead of time, just either:

• hide the preview entirely
• set it to all white, which would look the same as getting rid of it entirely in the light theme, and it'd probably still make sense in other themes

[chrismaltby]

Hi @coasterspaul I've just made an update to improve this in #596 which should hopefully work better now. I just need to do some testing before I merge that but from a quick try it seems to be working much better.

A keep-as-is feature would be quite nice, I think it might be a bit tricky with they way palettes are handled though.

[coasterspaul]

And yet you managed to add the keep-as-is feature to beta 4 anyway? Thanks so much!

[chrismaltby]

Hah, yep I figured out a way to do it without changing too much in the end :-D