Add ability to select top bar height when floating panel is enabled

[Impasse52]

I would like to be able to select the floating panel's height when the floating panel option is enabled.
I think it would be ideally put just under the "gap" option with the same kind of +/- buttons.

I'm also open to help with this (eventually implementing this myself), but I'm not familiar with how Gnome's themes handles CSS. If I could have resources about that, I'd be happy to help.