fix: use height_cm instead of width_cm
New themes panel.margin.x and panel.margin.y

Fixes #839

Conflicts:
	NEWS
	R/facet-grid-.r

Resolved trivially.
Roxygenize
Merge branch 'master' into feature/839-facet-margin-2
Merge branch 'master' into feature/839-facet-margin-2
Merge branch 'feature/839-facet-margin-2' of github.com:krlmlr/ggplot2 into krlmlr-feature/839-facet-margin-2

Conflicts:
	man/theme.Rd