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/package.json

@@ -83,8 +83,8 @@
             "properties": {
                 "klammertext.pythonPath": {
                     "type": "string",
-                    "default": "python3",
-                    "description": "Python interpreter used to run the Klammertext language server."
+                    "default": "",
+                    "description": "Python interpreter used to run the Klammertext language server.  Leave blank for python3 (on Windows: python)."
                 },
                 "klammertext.serverPath": {
                     "type": "string",