M4.57: PNG, PDF and text clipboard flavours in and out

Copy and Cut also offer other apps a PNG of the copied objects, a PDF
with On Copy: PDF, and plain text when every copied object is type
(clipboard.flavours; clipboard.exportPng {scale?}, exportPdf,
exportText). Paste outside text editing takes what another app copied:
SVG, then PDF (clipboard.importPdf), then plain text as point text
(clipboard.importText), then a bitmap embedded at the view centre
(clipboard.importImage {dataBase64, mime?, center?}); pasting our own
copy keeps the lossless internal clipboard, and Cmd+V pastes a bitmap
that comes with no text. Services gains system_clipboard
(SystemClipboard: write, holds_ours, read, has): on Windows every
format at once, on macOS and Linux one (the text, else the PNG); the
web keeps SVG text only. With When pasting text: Keep Plain Text, text
the Type tool copied pastes without its formatting.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Cargo.toml

@@ -56,6 +56,7 @@ eframe = { version = "0.36", default-features = false }
 egui_kittest = "0.36"
 rfd = "0.17"
 arboard = { version = "3.6", default-features = false }
+clipboard-win = "5.4"
 webbrowser = "1"
 
 [workspace.lints.rust]