Space between controls - AKA spacers

`libui-rs` currently uses empty `uiBox`es as stretchy spacers, but what if I want fixed spacers? Not by any particular amount, of course, I just want a _unit of space_ to separate things out by. Like the window margin - no nonstandard px amounts, just one standard unit of space.

Right now, an empty label works okay as a vertical spacer.


But what about a horizontal one? A label with a couple space characters as its content? Honestly, both of these solutions feel dirty to me. I think libui should implement proper spacers.

There could be a zero-size spacer, meant for stretchy use, and a one-size spacer meant for non-stretchy use. Maybe there could be two variants of the one-size spacer - one vertical one with zero width, and one horizontal one with zero height.