Customizable block palette opacity

[mxmou]

Changes

Allows the opacity of the block palette to be changed using editor dark mode. All presets use the default 80%. Migration code will be needed to add transparency to existing settings.

Reason for changes

I don't think there is a good reason to not make it customizable.

[WorldLanguages]

Good idea! This should be customizable

[WorldLanguages]

@apple502j Please review my changes to get-addon-settings.js Is it safe to remove the old transition code? Or do we still need to set addonSettings["editor-dark-mode"]._version = 2 if it's not yet defined?

[apple502j]

@WorldLanguages Read the comment on that file

[apple502j]

Or do we still need to set addonSettings["editor-dark-mode"]._version = 2 if it's not yet defined?

Yes

[apple502j]

#4817 changed the transition code to better handle no-op transition.