feat: canonically provide $ACT to javascript

This is a more consistent place relative to pageinfo()

New CLI Plugin Type

Better core search

PWA manifest and new @ini_theme_color replacement

json_encode is available natively since PHP 5.2

This should make their purpose more obvious.

They are now named min and max respectively, which is hopefully less
cryptic.

This is useful for anonymously useable forms, like the search.

Pressing enter in the small quick-search-box while it was empty resulted
in a search for the current pageid.

Since we no longer change the page during a search, using the "show"
menu item to return to the current page makes more sense.

This way they can be cleanly influenced by the plugins instead of having
to modify global state.

As suggested by @splitbrain SearchState now behaves simliar to the PSR-7
message interfaces.

Implements the functionality requested in #1124

This removes a lot of reudundant CSS classes in favor of cascading
selectors. Styling isn't done, yet this is just a first step.

Still to do:

* aria labels
* styling the toggle mechanism
* RTL styling
* making sure everything displays correct when no JS is available
* translations may be missing
* refactoring the rest of search.less to make use of LESS features

This also renames the event for the search form on the search page.

The scheme for naming events that provide the new Form-class as $data is
FORM_<formname>_OUTPUT

That parameter was introduced in cbcc2fa5 and removed in d22b78c8

If the entered query does not cleanly fall into the area of the given
options, show the current behavior as "custom".

This gives create control over when a query is too complex for tool
support.

This functionality is inspired by what other large search engines are
doing.