multiple inside guide box with different position

[Yunuuuu]

Try to fix #5712.

This enables users to set legend.position.inside and legend.justification.inside in theme of guide_legend and guide_colourbar.

Here is an example:

ggplot(mpg, aes(displ, hwy, shape = drv, colour = cty, size = year)) +
    geom_point(aes(alpha = cyl)) +
    guides(
        colour = guide_colourbar(
            position = "inside",
            theme = theme(legend.position.inside = c(1, 0))
        ),
        size = guide_legend(position = "top"),
        alpha = guide_legend(
            position = "inside",
            theme = theme(legend.position.inside = c(0, 1))
        ),
        shape = guide_legend(position = "left")
    )