Allow to uninstall a theme on the Appearance page

The functional implementation is postponed on #2232605: Themes cannot be uninstalled

However, I think we should kick-start some design + usability considerations already, because:

1. "Uninstall" is essentially a new link to be added to the list of operation links for each theme.
2. But those operation links are cluttering the interface already today.
3. Their dynamic behavior, depending on enabled/disabled + default/admin status, makes each link always different and thus hard to discover.

The following operations exist:

• Enable
• Enable and set as default
• Set as default
• Set as admin*
• Settings**
• Disable
• Uninstall

* "Set as admin" is not part of the operation links currently, but in a separate fieldset + select list instead (which looks weird). That is, because the operation is accompanied by a "Set no theme as admin theme" operation. FWIW, in technical terms, setting no theme as admin theme means the same as setting the default theme as admin theme.

** "Settings" is not an operation link currently. The settings for a particular theme are reached via (disconnected) tabs on the page.

The most commonly used operation is "Enable and set as default".

The sense and fate of "Disable" is unclear right now and to be figured out in #2250155: Rethink how former use-cases for disabled themes can be supported in D8

For consistency, it would be great if (almost) all operations were exposed for each theme in the same order, whereas inapplicable operations would be disabled/greyed-out.

Thinking of available patterns in core, perhaps we should consider to use a Dropbutton/split-button?