This adds labels to the icons and moves them away from the search box.
These changes are made together, because they work together, but are based on
several complaints:
* The [+/-] thing are a Reddit-ism. They don't look like buttons, but look
like syntax
<https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/More.20visual.20difference.20for.20the.20.2B.2F-.20.20Icons>,
<https://github.com/rust-lang/rust/issues/59851>
(some of these are laundry lists with more suggestions, but they all
mention [+/-] looking wrong)
* The settings, help, and summary buttons are also too hard to recognize
<https://lwn.net/Articles/987070/>,
<https://github.com/rust-lang/rust/issues/90310>,
<https://github.com/rust-lang/rust/issues/14475#issuecomment-274241997>,
<https://internals.rust-lang.org/t/improve-rustdoc-design/12758>
("Not all functionality is self-explanatory, for example the [+] button in
the top right corner, the theme picker or the settings button.")
The toggle-all and toggle-individual buttons both need done at once, since we
want them to look like they go together. This changes them from both being
[+/-] to both being arrows.
Settings and Help are also migrated, so that the whole group can benefit from
being described using actual words.
Additionally, the Help button is only shown on SERPs, not all the time.
This is done for two major reasons:
* Most of what's in there is search-related. The things that aren't are
keyboard commands, and the search box tells you about that anyway.
Pressing <kbd>?</kbd> will temporarily show the button and its popover.
* I'm trading it off by showing the help button, even on mobile.
It's useful since you can use the search engine suggestions there.
* The three buttons were causing line wrapping on too many desktop layouts.
Designing a good hover microinteraction is a matter of guessing
user intent from what are, literally, vague gestures. In this case,
guessing if hovering in our out of the tooltip base is intentional
or not.
To figure this out, a few different techniques are used:
* When the mouse pointer enters a tooltip anchor point, its hitbox
is grown on the bottom, where the popover is/will appear. This was
already there before this commit: search "hover tunnel" in
rustdoc.css for the implementation.
* This commit adds a delay when the mouse pointer enters the base
anchor, in case the mouse pointer was just passing through and the
user didn't want to open it.
* This commit also adds a delay when the mouse pointer exits the
tooltip's base anchor or its popover, before hiding it.
* A fade-out animation is layered onto the pointer exit delay to
immediately inform the user that they successfully dismissed the
popover, while still providing a way for them to cancel it if
it was a mistake and they still wanted to interact with it.
* No animation is used for revealing it, because we don't want
people to try to interact with an element while it's in the
middle of fading in: either they're allowed to interact with
it while it's fading in, meaning it can't serve as mistake-
proofing for opening the popover, or they can't, but they
might try and be frustrated.
See also:
* https://www.nngroup.com/articles/timing-exposing-content/
* https://www.nngroup.com/articles/tooltip-guidelines/
* https://bjk5.com/post/44698559168/breaking-down-amazons-mega-dropdown
