[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/history.cpp

@@ -6,10 +6,10 @@
  * PROGRAMMERS: Benedikt Freisen
  */
 
-/* INCLUDES *********************************************************/
-
 #include "precomp.h"
 
+ImageModel imageModel;
+
 /* FUNCTIONS ********************************************************/
 
 void ImageModel::NotifyDimensionsChanged()