zgui: Make buttons visually distinctive from labels

[ozkriff]

Atm, it's not clear if the widget is clickable or not.

nicoburns:
Some kind of button with an "active" state would be nice, so that you can more easily tell which bits are clickable.

(from https://www.reddit.com/r/rust/comments/ale980/a_wasm_version_of_my_tactical_game_zemeroth_is)

Related to #150

[ozkriff]

For now, I'm trying to stick to the convention of using [ and ] around the text of buttons, but it's a messy solution and won't work for image buttons anyway.