[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.h

@@ -23,6 +23,9 @@ public:
         MESSAGE_HANDLER(WM_GETTEXT, OnGetText)
     END_MSG_MAP()
 
+    HWND DoCreate();
+
+private:
     LRESULT OnCreate(UINT nMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled);
     LRESULT OnCloseOrKeyDownOrLButtonDown(UINT nMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled);
     LRESULT OnPaint(UINT nMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled);