Drops: libraries, presets and plug-ins open on a drop, as File › Open opens them

With a document open, the desktop app ran File › Place on a swatch or
graphic style library, a Libraries panel file, a presets file or a
plug-in dropped on it. A desktop drop has no position, and without one
the app opened only documents and ran File › Place on every other file.
On the web, the app also ran File › Place on these files when they were
dropped on the canvas. File › Place can't read them, and the drop
failed with "Couldn't place X: ... not a format VectorCraft reads".

VectorcraftApp::drop_target now returns Open for these files, with or
without a drop position, and they open as File › Open opens them:
plug-ins are installed, presets are imported, swatch and graphic style
libraries open in the library panel, and a Libraries panel file is
added to the Libraries panel as a new library. A dropped library,
presets file or plug-in that has a path is added to Open Recent Files
when it opens, as File › Open adds it. io::opens_as returns the kind of
each of these files from its extension, and drop_target and
io::open_bytes both call it. The rules from #472 still apply to
documents, pictures and text. On the web the browser gives no file name
before the drop, so the app still outlines the canvas while such a file
is dragged over it.

Tests: drop_target returns Open for each of the eleven extensions, with
no position and with a position on the canvas. A .gpl dropped through
the desktop drop path loads in the library panel and is added to Open
Recent Files, and a .vclibrary dropped the same way becomes a new
library in the Libraries panel.

Docs: docs/control-protocol.md (file.place), docs/development.md
(Drag-and-drop), docs/target-app-parity.md (Evidence, UI).

apps/vectorcraft-web/src/web.rs

@@ -308,7 +308,8 @@ fn notice(lines: &[&str]) {
 }
 
 /// Wraps the app to read dropped files asynchronously (browsers can't read them synchronously)
-/// and feed them through the inboxes: placed where they were dropped on the canvas, else opened.
+/// and feed them through the inboxes: placed where they were dropped on the canvas, else opened
+/// (libraries, presets and plug-ins open wherever they are dropped).
 /// When the graphics are lost it hands the app to a new runner on a new canvas.
 struct WebShell {
     /// `None` once handed on.