[Dialog] Add support for a Dialog without a DialogTitle

[megos]

• I have followed (at least) the PR section of the contributing guide.

Spacing is narrow when use dialog without title.

https://material.io/design/components/dialogs.html

before	after

Has no effect when with title.

[oliviertassinari]

Thanks this looks good. I have given it a second look to iron some other details.

[mui-pr-bot]

Details of bundle changes.

Comparing: 67f4657...5013002

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.02% 🔺	+0.01% 🔺	328,822	328,874	89,898	89,904
@material-ui/core/Paper	0.00%	0.00%	68,684	68,684	20,476	20,476
@material-ui/core/Paper.esm	0.00%	0.00%	62,058	62,058	19,206	19,206
@material-ui/core/Popper	0.00%	0.00%	28,468	28,468	10,177	10,177
@material-ui/core/Textarea	0.00%	0.00%	5,094	5,094	2,136	2,136
@material-ui/core/TrapFocus	0.00%	0.00%	3,834	3,834	1,614	1,614
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,386	16,386	5,826	5,826
@material-ui/core/useMediaQuery	0.00%	0.00%	2,541	2,541	1,059	1,059
@material-ui/lab	0.00%	0.00%	152,975	152,975	46,649	46,649
@material-ui/styles	0.00%	0.00%	51,401	51,401	15,290	15,290
@material-ui/system	0.00%	0.00%	15,658	15,658	4,361	4,361
Button	0.00%	0.00%	78,687	78,687	24,044	24,044
Modal	0.00%	0.00%	14,346	14,346	5,011	5,011
Portal	0.00%	0.00%	2,907	2,907	1,318	1,318
Rating	0.00%	0.00%	70,047	70,047	21,881	21,881
Slider	0.00%	0.00%	74,338	74,338	23,043	23,043
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	51,846	51,846	13,780	13,780
docs.main	0.00%	0.00%	590,557	590,557	188,449	188,449
packages/material-ui/build/umd/material-ui.production.min.js	+0.02% 🔺	+0.01% 🔺	299,821	299,873	86,253	86,260

Generated by 🚫 dangerJS against 5013002

[joshwooding]

@megos It's a great first contribution to Material-UI. 🎉It's always great to see us getting closer to the spec. Thanks for working on it :)