This PR aims tofix#5935.

Briefly, a new theme elementstrip.spacing is used to set the distance from the panel to the strip unconditionally.

Previously, we could only usestrip.switch.pad.{grid/wrap} to set this distance, but only if there was an axis in between the panel and the strip. In case the conditions for thestrip.switch.pad.{grid/wrap} are met, thenstrip.spacing is additive to that spacing. The default spacing isrel(0) inheriting from the roottheme(spacing) argument, so it doesn't add space by default.

However, it does add a gtable cell even when there is 0-spacing, which helps towards#5628 as the gtable shape no longer depends on the relative placement of axes and strips (the cell is added regardless).

To give a few examples. We weren't able to do this before:

devtools::load_all("~/packages/ggplot2")#> ℹ Loading ggplot2p<- ggplot(mpg, aes(displ,hwy))+  geom_point()p+ facet_wrap(year~drv)+  theme(strip.spacing= unit(1,"cm"))

Or this:

p+ facet_grid(year~drv,switch="both")+  theme(strip.spacing= unit(1,"cm"))

^{Created on 2024-06-27 withreprex v2.1.0}