Ensure Gallery widget is styled properly across widget areas in bundled themes

[westonruter]

The Gallery widget (#41914) allows for image galleries to be added to widget areas. Many themes may not have styles in them that expect galleries to appear in a widget context, since previously galleries would have only appeared in post content. As noted in §Default Themes Updates for ​Media Widgets for Images, Video, and Audio:

Themes that add custom styles to the MediaElement.js player (namely Twenty Thirteen and Twenty Fourteen) were updated from just styling it within syndicated content, to also include instances within widgets. Most themes don’t restrict styles for captioned images or media players to just post content, that is, limit CSS selectors to classes output by post_class(). If your theme does, make sure to either remove that constraint or include a .widget selector.

In the same way for the Gallery widget, we need to make sure that all of the core themes are compatible. Gallery widgets need to be tested in all core themes and in each of their widget areas, while also changing settings like the number of columns and the image size, in order to check for the variations.

Once the core themes are updated then the changes will may be instructive for theme authors to make similar changes in their themes.

Let's get some screenshots from each default theme, along with some style improvement suggestions :)

Gallery widgets in Twenty Seventeen

Gallery widgets in Twenty Sixteen

Gallery widgets in Twenty Fifteen

Gallery widgets in Twenty Fourteen

Gallery widgets in Twenty Thirteen

Gallery widgets in Twenty Twelve

Gallery widgets in Twenty Eleven (no sidebar)

Gallery widgets in sidebar in Twenty Eleven

Gallery widgets in Twenty Ten

I added screenshots. They mostly look good, but Twenty Thirteen has some obvious problems, and Twenty Ten has a little overlap.

See similar task for media added to Text widget: #42203

A little bit of CSS can fix some of the issues shown in twentythirteen theme. It doesn't fix them all (larger amounts of columns with non-thumbnail image configs still have clearing issues) but it covers many of the cases.

I will continue to work towards fixing any unusual quirks in the styling of these widgets across core themes as I have time.

Thanks @joyously for the testing and screenshots! Thanks @williampatton for the testing and patch!

I've been testing this today, and am seeing mostly the same stuff. I'm going to test a bit more tomorrow, check out the patch and see if anything else needs to be done.