[system] Add support for marginX, marginY, paddingX, and paddingY

[dimitropoulos]

I strongly prefer not using the "single letter" shorthand for clarity's sake. That said.. I do really like the mx, my, px, py concepts. I tried to change as little as possible to accomplish this. Thoughts?

[mui-pr-bot]

Details of bundle changes.

Comparing: 98292b4...55192bc

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.04% 🔺	+0.06% 🔺	318,737	318,854	87,520	87,571
@material-ui/core/Paper	0.00%	0.00%	68,280	68,280	20,356	20,356
@material-ui/core/Paper.esm	0.00%	0.00%	61,577	61,577	19,136	19,136
@material-ui/core/Popper	0.00%	0.00%	28,945	28,945	10,405	10,405
@material-ui/core/Textarea	0.00%	0.00%	5,513	5,513	2,373	2,373
@material-ui/core/TrapFocus	0.00%	0.00%	3,755	3,755	1,580	1,580
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,012	16,012	5,794	5,794
@material-ui/core/useMediaQuery	0.00%	0.00%	2,597	2,597	1,103	1,103
@material-ui/lab	0.00%	0.00%	140,015	140,015	43,325	43,325
@material-ui/styles	0.00%	0.00%	51,703	51,703	15,342	15,342
@material-ui/system	+0.76% 🔺	+1.22% 🔺	15,303	15,420	4,339	4,392
Button	0.00%	0.00%	84,278	84,278	25,631	25,631
Modal	0.00%	0.00%	20,330	20,330	6,664	6,664
Slider	0.00%	0.00%	74,703	74,703	23,219	23,219
colorManipulator	0.00%	0.00%	3,904	3,904	1,544	1,544
docs.landing	0.00%	0.00%	55,232	55,232	13,946	13,946
docs.main	0.00%	0.00%	650,902	650,902	205,053	205,053
packages/material-ui/build/umd/material-ui.production.min.js	+0.04% 🔺	+0.06% 🔺	292,060	292,180	83,522	83,573

Generated by 🚫 dangerJS against 55192bc

[eps1lon]

The switch statement you introduce needs a no-op default case. I'm not a fan of aliases though I'm not a fan of the whole API any. @oliviertassinari should make the call here.

[dimitropoulos]

@eps1lon I agree... if you would like me to make a PR to undo the shorthand syntax (the 2 letter one) I'd be more than happy! :)

[oliviertassinari]

I like the idea for consistency with the mx, my, px, py props. I think that we can move forward. We need to update the tests, the documentation. Regarding the implementation, we can probably improve it.

[oliviertassinari]

@dimitropoulos I have tried the following 9456f78 (12 insertions(+), 21 deletions(-)) what do you think about it?

[eps1lon]

@dimitropoulos It's a great first pull request on Material-UI 👌🏻. Thank you for working on it!