1.0.6: Let a manifest say where a plugin belongs and what it looks like

Two fields the panel's plugin list now reads. "category" is one word from a
fixed vocabulary and becomes the tab a plugin is found under; a word the panel
does not know is filed under other rather than refused, which is the bargain
the rest of the manifest already strikes. "screenshots" is up to six pictures
shipped inside the plugin, optionally captioned.

A screenshot is a file in the plugin: png, jpg, webp, avif or gif, on a
relative path that stays inside it. Not SVG, which can carry script, and never
a remote URL - the panel serves these itself rather than sending every
administrator's browser to fetch an address a manifest chose.

Say nothing and the panel still looks in screenshots/ and then at a
screenshot.png in the root, so a repository that already has one needs no
change. Themes get the paragraph they need: the list draws a theme as its
picture, because what a theme is cannot be written in a sentence, and a theme
that names no category is filed under appearance.

The docs also now say what the panel will not read from a manifest: who wrote
the plugin. That badge comes from a list the panel ships.