Palette control for `SimpleEffectDialog`

[Lehonti]

In some effects, the user may want to define custom color palettes. At the moment, an acceptable workaround is having an enum with pre-defined palettes (see #457), but it would be nice to have something better, both at the GUI level and at the API level.

One idea is having a "compact" mode that allows us to get a glimpse of the colors of the palette, and an 'edit' mode (for example, a pop-up window) that allows us to edit it. Let's not forget that a palette can have many colors and it may not be practical to show them as controls in SimpleEffectDialog.