Add multi-window support to GUI with shared buffer list and improved input handling

- Introduced support for multiple windows, sharing the primary editor's buffer list.
- Added `GUIFrontend::OpenNewWindow_` for creating secondary windows with independent dimensions and input handlers.
- Redesigned `WindowState` to encapsulate per-window attributes (dimensions, renderer, input, etc.).
- Updated input processing and command execution to route events based on active window, preserving window-level states.
- Enhanced SDL2 and ImGui integration for proper context management across multiple windows.
- Increased robustness by handling window closing, resizing, and cleanup of secondary windows without affecting the primary editor.
- Updated documentation and key bindings for multi-window operations (e.g., Cmd+N / Ctrl+Shift+N).
- Version updated to 1.8.0 to reflect the major GUI enhancement.

Command.h

@@ -111,6 +111,8 @@ enum class CommandId {
 	SetOption, // generic ":set key=value" (v1: filetype=<lang>)
 	// Viewport control
 	CenterOnCursor, // center the viewport on the current cursor line (C-k k)
+	// GUI: open a new editor window sharing the same buffer list
+	NewWindow,
 };