Clean up stale editor-hInstance comments, add isOwnModulePath edge tests

Drops a false claim about ReaSampler's own editor's hInstance, corrects the real reason for registering the wndproc class against our own module, and covers isOwnModulePath's separator edge cases.

src/shell/actions/drag_out_win.cpp

@@ -378,8 +378,8 @@ ui::WindowFacts windowFacts(HWND hwnd) {
 
 bool pointerOverPluginEditor(int screenX, int screenY) {
     // The CLASS procedure's module rather than the window's hInstance: a bare-SDK plugin may
-    // create its editor with the hInstance of the parent REAPER hands it (ReaSampler 9000's own
-    // editor does), so hInstance would read that plugin as REAPER.
+    // create its editor with the hInstance of the parent REAPER hands it, so hInstance would
+    // read that plugin as REAPER.
     const POINT pt{screenX, screenY};
     const HWND hit = WindowFromPoint(pt);
     if (!hit) return false;