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.

mac/file.cpp

@@ -72,14 +72,17 @@ size_t count_substrings(const std::string& text, const std::string& substring) {
 
 std::string home_directory()
 {
-    if (const char* home = std::getenv("HOME"); home && *home) {
-        return home;
-    }
 #ifdef _WIN32
+    // USERPROFILE first: an MSYS2 shell may set HOME to its own /home/<user>
+    // (C:\msys64\home\<user>), and a Windows user's ~/.klammertext must be
+    // the same directory from PowerShell and from that shell.
     if (const char* profile = std::getenv("USERPROFILE"); profile && *profile) {
         return profile;
     }
 #endif
+    if (const char* home = std::getenv("HOME"); home && *home) {
+        return home;
+    }
     return {};
 }