[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/toolsettings.cpp

@@ -24,11 +24,12 @@
 
 static const BYTE s_AirRadius[4] = { 5, 8, 3, 12 };
 
+CToolSettingsWindow toolSettingsWindow;
+
 /* FUNCTIONS ********************************************************/
 
 BOOL CToolSettingsWindow::DoCreate(HWND hwndParent)
 {
-    /* creating the tool settings child window */
     RECT toolSettingsWindowPos =
     {
         X_TOOLSETTINGS, Y_TOOLSETTINGS,