Theme grid order and panel line width

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• closes Possible improvements for panel border #1407
• adds Theme(grid_line_order=0)
• adds Theme(panel_line_width=0.3mm)
• docstrings added to Theme

Example:

In this example, the grid lines are behind the panel frame.

plot(x=[5100, 5400], y=[0, 1], Geom.blank,
    Guide.yticks(ticks=0:0.1:1),
    Theme(grid_line_style=:solid, grid_line_width=0.5mm,
        grid_color="gray92", panel_stroke="black", 
        grid_line_order=-2,  panel_line_width=1mm)
)

[Mattriks]

Tests are passing now with RDatasets 0.6.7. For this PR, I just need to check off the regression tests.

[Mattriks]

The regression test pass. The only other testscript to use panel_stroke is "subplot_layers.jl". For "subplot_layers.jl" there is a difference in the panel border (slightly thinner for devel-output compared to master-output - for master the panel border must be inheriting it's linewidth from somewhere, because it's not explicitly set). But I've tested this PR with Geom.subplot_grid, and the new features panel_line_width and grid_line_order behave as expected. So this PR is ready to merge.

[Mattriks]

bump @bjarthur