[MSPAINT][ATL] Encapsulation: mainWindow (#5178)

- Add DoCreate methods to CFullscreenWindow, CMiniatureWindow, and CMainWindow classes.
- Do encapsulation around mainWindow and _tWinMain.
- Add GetOpenFileName, GetSaveFileName, and ChooseColor helper methods to CMainWindow class.
- Move some code in WinMain into CMainWindow::OnCreate.
- Delay creation of CFullscreenWindow and CMiniatureWindow.
- Extend ATL CImage class as CImageDx in newly-created atlimagedx.h of mspaint.
CORE-18867

base/applications/mspaint/fullscreen.cpp

@@ -6,12 +6,21 @@
  * PROGRAMMERS: Benedikt Freisen
  */
 
-/* INCLUDES *********************************************************/
-
 #include "precomp.h"
 
+CFullscreenWindow fullscreenWindow;
+
 /* FUNCTIONS ********************************************************/
 
+HWND CFullscreenWindow::DoCreate()
+{
+    if (m_hWnd)
+        return m_hWnd;
+
+    RECT rc = {0, 0, 0, 0}; // Rely on SW_SHOWMAXIMIZED
+    return Create(HWND_DESKTOP, rc, NULL, WS_POPUPWINDOW, WS_EX_TOPMOST);
+}
+
 LRESULT CFullscreenWindow::OnCreate(UINT nMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled)
 {
     SendMessage(WM_SETICON, ICON_BIG, (LPARAM) LoadIcon(hProgInstance, MAKEINTRESOURCE(IDI_APPICON)));