Add option to set custom spacing variables

[git-f0x]

This would enable setting spacing values different than the presets provided by the Compact, Comfortable and Spacious Interface Density settings. Changing the theme spacing config files manually doesn't seem to update the UI, so the app could trigger a theme update (similar to Settings).

[Thatoo]

As panel is divided in three, left, central and right, it would be nice to be able to set different spacing between icons independently for the three segments and allow to go to a very minimal spacing between applet's icons : right now, 0 is good between apps icons but, on small small laptop screen, is still too much space between applet's icons.

I also opened an issue here https://github.com/pop-os/cosmic-panel/issues/304