Add default horizontal & vertical padding to theme

[cmyr]

This also uses these values in the Flex container, via new
add_default_spacer and with_default_spacer methods. This should
be the preferred way of spacing the standard control widgets.

This is progress on #1216; I also want to go and remove any other
internal padding constants from existing widgets.

[luleyleo]

Seems like a good idea.

Maybe with_theme_spacer would be a more obvious name? Because I have no concept of a "default spacer" but I would immediately know what's meant by a "theme spacer".

[cmyr]

I was thinking about making this with_spacer and making with_spacer be with_fixed_spacer, but that would be annoying breakage. maybe with_auto_spacer?

[luleyleo]

Hmm, I've the same problem with with_auto_spacer as with with_default_spacer, actually I've even more of an issue with 'auto' because it makes me think of something adaptive...

Actually, now that I think about it again, with_default_spacer seems fine to me. Calling it with_theme_spacer makes no sense if the user (or even Druid) would define multiple different "default" sizes.

Maybe we could call them with_spacer and with_custom_spacer?. I don't think changing with_spacer would be an issue, as it is a single find-and-replace to fix it.

This is one of the rather rare moments where I wish Rust had overloading...

[cmyr]

I'm not sure there's anything I like better than with_default_spacer, so I think I'm just going to keep that as-is for now 🤷