Enable inheritance for derived theme elements

[clauswilke]

This closes #3549.

library(ggplot2)

# like element_rect but with a meaningless dummy argument
element_dummyrect <- function(dummy) {
  structure(list(
    fill = NULL, colour = NULL, dummy = dummy, size = NULL,
    linetype = NULL, inherit.blank = FALSE
  ), class = c("element_dummyrect", "element_rect", "element"))
}

ggplot2:::calc_element(
  "panel.background",
  theme(
    rect = element_rect(fill = "white", colour = "black", size = 0.5, linetype = 1),
    panel.background = element_dummyrect(dummy = 5))
)
#> List of 6
#>  $ fill         : chr "white"
#>  $ colour       : chr "black"
#>  $ dummy        : num 5
#>  $ size         : num 0.5
#>  $ linetype     : num 1
#>  $ inherit.blank: logi FALSE
#>  - attr(*, "class")= chr [1:3] "element_dummyrect" "element_rect" "element"

^{Created on 2019-10-04 by the reprex package (v0.3.0)}

[clauswilke]

@thomasp85 Could you take a look at this? It would help me to have this fix in master. Thanks!

[thomasp85]

LGTM

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