Windows: product names, the editors, WSL; the Linux page's image step

Assembled from development commit c3c40f6c8fca. Since 830b193: on Windows the
products carry Windows names (ktext.exe, klammertext.dll, document.dll,
reflect.dll) and the console's output code page is UTF-8 for a run; the
editor tooling works on Windows (VS Code, Sublime Text, Vim, with the
interpreter's name chosen by platform) and Notepad++ joins it
(doc/edit/notepadpp: a User Defined Language and two Run-menu commands);
a sixth install page, doc/install/windows_wsl_install.md, for Linux
inside WSL; the Windows page opens with the three routes and gains its
editor section, the PowerShell quoting and Path notes; the Linux page
installs OpenImageIO from the python3-openimageio package and verifies
with kdiag --machine; the README lists all six install pages. Linux and
macOS behaviour is unchanged.

doc/edit/vscode/extension.js

@@ -165,7 +165,11 @@ function activate(context) {
             'diagnostics, formatting, matching and alignment need the server.');
         return;
     }
-    const python = vscode.workspace.getConfiguration('klammertext').get('pythonPath') || 'python3';
+    // The setting's declared default is empty so that this fallback is what
+    // decides: python3 everywhere but Windows, where python.org installs
+    // "python" only (the Klammertext commands there use the same name).
+    const python = vscode.workspace.getConfiguration('klammertext').get('pythonPath') ||
+        (process.platform === 'win32' ? 'python' : 'python3');
     client = new LspClient(python, [serverPath], log);
     client.proc.on('error', (err) => {
         vscode.window.showWarningMessage(